The document at https://tc39.es/ecma262/ is the most accurate and
up-to-date ECMAScript specification. It contains the content of the most recent yearly snapshot plus any
finished proposals
(those that have reached Stage 4 in the proposal
process and thus are implemented in several implementations and will be in the next practical
revision) since that snapshot was taken.
This specification is developed on GitHub with the help of the ECMAScript community. There are a number
of ways to contribute to the development of this specification:
Refer to the colophon
for more information on how this document is created.
Introduction
This Ecma Standard defines the ECMAScript 2026 Language. It is the seventeenth edition of the ECMAScript
Language Specification. Since publication of the first edition in 1997, ECMAScript has grown to be one
of the world's most widely used general-purpose programming languages. It is best known as the language
embedded in web browsers but has also been widely adopted for server and embedded applications.
ECMAScript is based on several originating technologies, the most well-known being JavaScript (Netscape)
and JScript (Microsoft). The language was invented by Brendan Eich at Netscape and first appeared in
that company's Navigator 2.0 browser. It has appeared in all subsequent browsers from Netscape and in
all browsers from Microsoft starting with Internet Explorer 3.0.
The development of the ECMAScript Language Specification started in November 1996. The first edition of
this Ecma Standard was adopted by the Ecma General Assembly of June 1997.
That Ecma Standard was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and
approved as international standard ISO/IEC 16262, in April 1998. The Ecma General Assembly of June 1998
approved the second edition of ECMA-262 to keep it fully aligned with ISO/IEC 16262. Changes between the
first and the second edition are editorial in nature.
The third edition of the Standard introduced powerful regular expressions, better string handling, new
control statements, try/catch exception handling, tighter definition of errors, formatting for numeric
output and minor changes in anticipation of future language growth. The third edition of the ECMAScript
standard was adopted by the Ecma General Assembly of December 1999 and published as ISO/IEC 16262:2002
in June 2002.
After publication of the third edition, ECMAScript achieved massive adoption in conjunction with the
World Wide Web where it has become the programming language that is supported by essentially all web
browsers. Significant work was done to develop a fourth edition of ECMAScript. However, that work was
not completed and not published as the fourth edition of ECMAScript but some of it was incorporated into
the development of the sixth edition.
The fifth edition of ECMAScript (published as ECMA-262 5th edition) codified de facto
interpretations of the language specification that have become common among browser implementations and
added support for new features that had emerged since the publication of the third edition. Such
features include accessor
properties, reflective creation and inspection of objects, program control of
property attributes, additional array manipulation functions, support for the JSON object encoding
format, and a strict mode that provides enhanced error checking and program security. The fifth edition
was adopted by the Ecma General Assembly of December 2009.
The fifth edition was submitted to ISO/IEC JTC 1 for adoption under the fast-track procedure, and
approved as international standard ISO/IEC 16262:2011. Edition 5.1 of the ECMAScript Standard
incorporated minor corrections and is the same text as ISO/IEC 16262:2011. The 5.1 Edition was adopted
by the Ecma General Assembly of June 2011.
Focused development of the sixth edition started in 2009, as the fifth edition was being prepared for
publication. However, this was preceded by significant experimentation and language enhancement design
efforts dating to the publication of the third edition in 1999. In a very real sense, the completion of
the sixth edition is the culmination of a fifteen year effort. The goals for this edition included
providing better support for large applications, library creation, and for use of ECMAScript as a
compilation target for other languages. Some of its major enhancements included modules, class
declarations, lexical block scoping, iterators and generators, promises for
asynchronous programming, destructuring patterns, and proper tail calls. The ECMAScript library of
built-ins was expanded to support additional data abstractions including maps, sets, and arrays of
binary numeric values as well as additional support for Unicode supplementary characters in strings and
regular expressions. The built-ins were also made extensible via subclassing. The sixth edition provides
the foundation for regular, incremental language and library enhancements. The sixth edition was adopted
by the General Assembly of June 2015.
ECMAScript 2016 was the first ECMAScript edition released under Ecma TC39's new yearly release cadence
and open development process. A plain-text source document was built from the ECMAScript 2015 source
document to serve as the base for further development entirely on GitHub. Over the year of this
standard's development, hundreds of pull requests and issues were filed representing thousands of bug
fixes, editorial fixes and other improvements. Additionally, numerous software tools were developed to
aid in this effort including Ecmarkup, Ecmarkdown, and Grammarkdown. ES2016 also included support for a
new exponentiation operator and adds a new method to Array.prototype called
includes.
ECMAScript 2017 introduced Async Functions, Shared Memory, and Atomics along with smaller language and
library enhancements, bug fixes, and editorial updates. Async functions improve the asynchronous
programming experience by providing syntax for promise-returning functions. Shared Memory and Atomics
introduce a new memory
model that allows multi-agent programs to communicate using atomic operations that ensure a
well-defined execution order even on parallel CPUs. It also included new static methods on Object:
Object.values, Object.entries, and
Object.getOwnPropertyDescriptors.
ECMAScript 2018 introduced support for asynchronous iteration via the async
iterator protocol and async generators. It also included four new regular
expression features: the dotAll flag, named capture groups, Unicode property escapes, and
look-behind assertions. Lastly it included object rest and spread properties.
ECMAScript 2019 introduced a few new built-in functions: flat and flatMap on
Array.prototype for flattening arrays, Object.fromEntries for directly turning
the return value of Object.entries into a new Object, and trimStart and
trimEnd on String.prototype as better-named alternatives to the widely
implemented but non-standard String.prototype.trimLeft and trimRight
built-ins. In addition, it included a few minor updates to syntax and semantics. Updated syntax included
optional catch binding parameters and allowing U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR)
in string literals to align with JSON. Other updates included requiring that
Array.prototype.sort be a stable sort, requiring that JSON.stringify return
well-formed UTF-8 regardless of input, and clarifying Function.prototype.toString by
requiring that it either return the corresponding original source text or a standard placeholder.
ECMAScript 2020, the 11th edition, introduced the matchAll method for Strings, to
produce an iterator for all match objects generated by a
global regular expression; import(), a syntax to asynchronously import Modules with a
dynamic specifier; BigInt, a new number primitive for working with arbitrary precision
integers;
Promise.allSettled, a new Promise combinator that does not short-circuit;
globalThis, a universal way to access the global this value; dedicated
export * as ns from 'module' syntax for use within modules; increased standardization of
for-in enumeration order; import.meta, a host-populated object available in Modules that may
contain contextual information about the Module; as well as adding two new syntax features to improve
working with “nullish” values (undefined or null): nullish
coalescing, a value selection operator; and optional chaining, a property access and function invocation
operator that short-circuits if the value to access/invoke is nullish.
ECMAScript 2021, the 12th edition, introduced the replaceAll method for Strings;
Promise.any, a Promise combinator that short-circuits when an input value is fulfilled;
AggregateError, a new Error type to represent multiple errors at once; logical assignment
operators (??=, &&=, ||=); WeakRef, for
referring to a target object without preserving it from garbage collection, and
FinalizationRegistry, to manage registration and unregistration of cleanup operations
performed when target objects are garbage collected; separators for numeric literals
(1_000); and Array.prototype.sort was made more precise, reducing the amount
of cases that result in an implementation-definedsort order.
ECMAScript 2022, the 13th edition, introduced top-level await, allowing the
keyword to be used at the top level of
modules; new class elements: public and private instance fields, public and private static fields,
private instance methods and accessors, and private static methods and accessors; static blocks inside
classes, to perform per-class evaluation initialization; the #x in obj syntax, to test for
presence of private fields on objects; regular expression match indices via the /d flag,
which provides start and end indices for matched substrings; the cause property on
Error objects, which can be used to record a causation chain in errors; the at
method for Strings, Arrays, and TypedArrays, which allows relative indexing; and
Object.hasOwn, a convenient alternative to Object.prototype.hasOwnProperty.
ECMAScript 2023, the 14th edition, introduced the toSorted,
toReversed, with, findLast, and findLastIndex
methods on Array.prototype and TypedArray.prototype, as well as the
toSpliced method on Array.prototype; added support for #!
comments at the beginning of files to better facilitate executable ECMAScript files; and allowed the use
of most Symbols as keys in weak collections.
ECMAScript 2024, the 15th edition, added facilities for resizing and transferring ArrayBuffers
and SharedArrayBuffers; added a new RegExp /v flag for creating RegExps with more advanced
features for working with sets of strings; and introduced the Promise.withResolvers
convenience method for constructing Promises, the Object.groupBy and
Map.groupBy methods for aggregating data, the Atomics.waitAsync method for
asynchronously waiting for a change to shared memory, and the String.prototype.isWellFormed
and String.prototype.toWellFormed methods for checking and ensuring that strings contain
only well-formed Unicode.
ECMAScript 2025, the 16th edition, added a new Iterator global with associated
static and prototype methods for working with iterators; added methods to
Set.prototype for performing common operations on Sets; added support for importing JSON
modules as well as syntax for declaring attributes of imported modules; added the
RegExp.escape method for escaping a string to be safely used in a regular expression; added
syntax for enabling and disabling modifier flags inline within regular expressions; added the
Promise.try method for calling functions which may or may not return a Promise
and ensuring the result is always a Promise; and added a new Float16ArrayTypedArray kind as well
as the related DataView.prototype.getFloat16, DataView.prototype.setFloat16,
and Math.f16round methods.
Dozens of individuals representing many organizations have made very significant contributions within
Ecma TC39 to the development of this edition and to the prior editions. In addition, a vibrant community
has emerged supporting TC39's ECMAScript efforts. This community has reviewed numerous drafts, filed
thousands of bug reports, performed implementation experiments, contributed test suites, and educated
the world-wide developer community about ECMAScript. Unfortunately, it is impossible to identify and
acknowledge every person and organization who has contributed to this effort.
Allen Wirfs-Brock
ECMA-262, Project Editor, 6th Edition
Brian Terlson
ECMA-262, Project Editor, 7th through 10th Editions
Jordan Harband
ECMA-262, Project Editor, 10th through 12th Editions
Shu-yu Guo
ECMA-262, Project Editor, 12th through 16th Editions
Michael Ficarra
ECMA-262, Project Editor, 12th through 16th Editions
Kevin Gibbons
ECMA-262, Project Editor, 12th through 16th Editions
1 Scope
This Standard defines the ECMAScript 2026 general-purpose programming language.
2 Conformance
A conforming implementation of ECMAScript must provide and support all the types, values, objects,
properties, functions, and program syntax and semantics described in this specification.
A conforming implementation of ECMAScript must interpret source text input in conformance with the latest
version of the Unicode Standard and ISO/IEC 10646.
A conforming implementation of ECMAScript that provides an application programming interface (API) that
supports programs that need to adapt to the linguistic and cultural conventions used by different human
languages and countries must implement the interface defined by the most recent edition of ECMA-402 that
is compatible with this specification.
A conforming implementation of ECMAScript may provide additional types, values, objects, properties, and
functions beyond those described in this specification. In particular, a conforming implementation of
ECMAScript may provide properties not described in this specification, and values for those properties,
for objects that are described in this specification.
A conforming implementation of ECMAScript may support program and regular expression syntax not described
in this specification. In particular, a conforming implementation of ECMAScript may support program
syntax that makes use of any “future reserved words” noted in subclause
12.7.2 of this specification.
A conforming implementation of ECMAScript must not implement any extension that is listed as a Forbidden
Extension in subclause 17.1.
A conforming implementation of ECMAScript may choose to implement or not implement Normative Optional subclauses. If any Normative Optional behaviour is
implemented, all of the behaviour in the containing Normative Optional clause must be implemented. A
Normative Optional clause is denoted in this specification with the words "Normative Optional" in a
coloured box, as shown below.
A conforming implementation of ECMAScript must implement Legacy subclauses,
unless they are also marked as Normative Optional. All of the language features and behaviours specified
within Legacy subclauses have one or more undesirable characteristics. However, their continued usage in
existing applications prevents their removal from this specification. These features are not considered
part of the core ECMAScript language. Programmers should not use or assume the existence of these
features and behaviours when writing new ECMAScript code.
2.3 Example Legacy Normative Optional Clause Heading
Example clause contents.
3 Normative References
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced
document (including any amendments) applies.
IEEE
754-2019, IEEE Standard for Floating-Point Arithmetic.
ISO/IEC 10646, Information Technology — Universal Multiple-Octet Coded Character Set (UCS) plus
Amendment 1:2005, Amendment 2:2006, Amendment 3:2008, Amendment 4:2008, and additional amendments and
corrigenda, or successor.
This section contains a non-normative overview of the ECMAScript language.
ECMAScript is an object-oriented programming language for performing computations and manipulating
computational objects within a host environment. ECMAScript as defined here is not
intended to be computationally self-sufficient; indeed, there are no provisions in this specification
for input of external data or output of computed results. Instead, it is expected that the computational
environment of an ECMAScript program will provide not only the objects and other facilities described in
this specification but also certain environment-specific objects, whose description and behaviour are
beyond the scope of this specification except to indicate that they may provide certain properties that
can be accessed and certain functions that can be called from an ECMAScript program.
ECMAScript was originally designed to be used as a scripting language, but has become widely used as a
general-purpose programming language. A scripting language is a programming language that is
used to manipulate, customize, and automate the facilities of an existing system. In such systems,
useful functionality is already available through a user interface, and the scripting language is a
mechanism for exposing that functionality to program control. In this way, the existing system is said
to provide a host
environment of objects and facilities, which completes the capabilities of the
scripting language. A scripting language is intended for use by both professional and non-professional
programmers.
ECMAScript was originally designed to be a Web scripting language, providing a mechanism to
enliven Web pages in browsers and to perform server computation as part of a Web-based client-server
architecture. ECMAScript is now used to provide core scripting capabilities for a variety of host
environments. Therefore the core language is specified in this document apart
from any particular host
environment.
ECMAScript usage has moved beyond simple scripting and it is now used for the full spectrum of
programming tasks in many different environments and scales. As the usage of ECMAScript has expanded, so
have the features and facilities it provides. ECMAScript is now a fully featured general-purpose
programming language.
4.1 Web Scripting
A web browser provides an ECMAScript host environment for client-side computation
including, for instance, objects that represent windows, menus, pop-ups, dialog boxes, text areas,
anchors, frames, history, cookies, and input/output. Further, the host environment provides a means to
attach scripting code to events such as change of focus, page and image loading, unloading, error
and abort, selection, form submission, and mouse actions. Scripting code appears within the HTML and
the displayed page is a combination of user interface elements and fixed and computed text and
images. The scripting code is reactive to user interaction, and there is no need for a main program.
A web server provides a different host environment for server-side computation
including objects representing requests, clients, and files; and mechanisms to lock and share data.
By using browser-side and server-side scripting together, it is possible to distribute computation
between the client and server while providing a customized user interface for a Web-based
application.
Each Web browser and server that supports ECMAScript supplies its own host
environment, completing the ECMAScript execution environment.
4.2 Hosts and Implementations
To aid integrating ECMAScript into host environments, this specification defers the
definition of certain facilities (e.g., abstract operations),
either in whole or in part, to a source outside of this specification. Editorially, this
specification distinguishes the following kinds of deferrals.
An implementation is an external source that further defines facilities
enumerated in Annex D or those that are marked as implementation-defined or implementation-approximated. In informal
use, an implementation refers to a concrete artefact, such as a particular web browser.
An implementation-defined facility is one that
defers its definition to an external source without further qualification. This specification does
not make any recommendations for particular behaviours, and conforming implementations are free to
choose any behaviour within the constraints put forth by this specification.
An implementation-approximated facility is
one that defers its definition to an external source while recommending an ideal behaviour. While
conforming implementations are free to choose any behaviour within the constraints put forth by this
specification, they are encouraged to strive to approximate the ideal. Some mathematical operations,
such as Math.exp, are implementation-approximated.
A host is an external source that further defines
facilities listed in Annex D but does not further define other
implementation-defined or implementation-approximated facilities.
In informal use, a host refers
to the set of all implementations, such as the set of all web browsers, that interface with this
specification in the same way via Annex D. A host is often an external specification, such as
WHATWG HTML (https://html.spec.whatwg.org/). In other
words, facilities that are host-defined are often further defined in external
specifications.
A host hook is an abstract operation
that is defined in whole or in part by an external source. All host hooks must be listed in Annex D. A host hook must conform to at least the
following requirements:
A host-defined facility is one that defers its definition
to an external source without further qualification and is listed in Annex D. Implementations that are not hosts may also provide definitions
for host-defined facilities.
A host environment is a
particular choice of definition for all host-defined facilities. A host
environment typically includes objects or functions which allow obtaining
input and providing output as host-defined properties of the global
object.
This specification follows the editorial convention of always using the most specific term. For
example, if a facility is host-defined, it should not be referred to as implementation-defined.
Both hosts and implementations
may interface with this specification via the language types, specification types, abstract operations,
grammar productions, intrinsic objects, and intrinsic symbols defined herein.
4.3 ECMAScript Overview
The following is an informal overview of ECMAScript—not all parts of the language are described. This
overview is not part of the standard proper.
ECMAScript is object-based: basic language and host facilities are provided by objects, and an ECMAScript
program is a cluster of communicating objects. In ECMAScript, an object is a collection of
zero or more properties each with attributes that determine how each property can
be used—for example, when the Writable attribute for a property is set to false,
any attempt by executed ECMAScript code to assign a different value to the property fails.
Properties are containers that hold other objects, primitive values, or functions.
A primitive value is a member of one of the following built-in types: Undefined, Null,
Boolean, Number, BigInt, String, and Symbol; an object is a
member of the built-in type Object; and a function is a callable object. A function that is
associated with an object via a property is called a method.
ECMAScript defines a collection of built-in objects that round out the definition of
ECMAScript entities. These built-in objects include the global object; objects that are
fundamental to the runtime semantics of the language including
Object, Function, Boolean, Symbol, and various
Error objects; objects that represent and manipulate numeric values including
Math, Number, and Date; the text processing objects
String and RegExp; objects that are indexed collections of values
including Array and nine different kinds of Typed Arrays whose elements all have a
specific numeric data representation; keyed collections including Map and
Set objects; objects supporting structured data including the JSON object,
ArrayBuffer, SharedArrayBuffer, and DataView; objects
supporting control abstractions including generator functions and Promise objects; and
reflection objects including Proxy and Reflect.
ECMAScript also defines a set of built-in operators. ECMAScript operators include various
unary operations, multiplicative operators, additive operators, bitwise shift operators, relational
operators, equality operators, binary bitwise operators, binary logical operators, assignment
operators, and the comma operator.
Large ECMAScript programs are supported by modules which allow a program to be divided into
multiple sequences of statements and declarations. Each module explicitly identifies declarations it
uses that need to be provided by other modules and which of its declarations are available for use
by other modules.
ECMAScript syntax intentionally resembles Java syntax. ECMAScript syntax is relaxed to enable it to
serve as an easy-to-use scripting language. For example, a variable is not required to have its type
declared nor are types associated with properties, and defined functions are not required to have
their declarations appear textually before calls to them.
4.3.1 Objects
Even though ECMAScript includes syntax for class definitions, ECMAScript objects are not
fundamentally class-based such as those in C++, Smalltalk, or Java. Instead objects may be
created in various ways including via a literal notation or via constructors which create objects and then
execute code that initializes all or part of them by assigning initial values to their
properties. Each constructor is a function that has a property named
"prototype" that is used to implement prototype-based inheritance
and shared properties. Objects are created by using constructors in new expressions;
for example, new Date(2009, 11) creates a new Date object. Invoking a constructor without
using new has consequences that depend on the constructor. For example,
Date() produces a string representation of the current date and time rather than an
object.
Every object created by a constructor has an implicit reference (called the
object's prototype) to the value of its constructor's "prototype"
property. Furthermore, a prototype may have a non-null implicit reference to
its prototype, and so on; this is called the prototype chain. When a reference is made
to a property in an object, that reference is to the property of that name in the first object
in the prototype chain that contains a property of that name. In other words, first the object
mentioned directly is examined for such a property; if that object contains the named property,
that is the property to which the reference refers; if that object does not contain the named
property, the prototype for that object is examined next; and so on.
Figure 1: Object/Prototype Relationships
In a class-based object-oriented language, in general, state is carried by instances, methods are
carried by classes, and inheritance is only of structure and behaviour. In ECMAScript, the state
and methods are carried by objects, while structure, behaviour, and state are all inherited.
All objects that do not directly contain a particular property that their prototype contains
share that property and its value. Figure 1 illustrates this:
CF is a constructor (and also an object). Five objects have
been created by using new expressions: cf1,
cf2, cf3, cf4, and cf5.
Each of these objects contains properties named "q1" and
"q2". The dashed lines represent the implicit prototype relationship; so, for
example, cf3's prototype is CFp. The constructor,
CF, has two properties itself, named "P1" and "P2",
which are not visible to CFp, cf1, cf2,
cf3, cf4, or cf5. The property named
"CFP1" in CFp is shared by cf1,
cf2, cf3, cf4, and cf5
(but not by CF), as are any properties found in CFp's implicit
prototype chain that are not named "q1", "q2", or
"CFP1". Notice that there is no implicit prototype link between CF and
CFp.
Unlike most class-based object languages, properties can be added to objects dynamically by
assigning values to them. That is, constructors are not required to name or assign
values to all or any of the constructed object's properties. In the above diagram, one could add
a new shared property for cf1, cf2, cf3,
cf4, and cf5 by assigning a new value to the property in
CFp.
Although ECMAScript objects are not inherently class-based, it is often convenient to define
class-like abstractions based upon a common pattern of constructor functions, prototype
objects, and methods. The ECMAScript built-in objects themselves follow such a class-like
pattern. Beginning with ECMAScript 2015, the ECMAScript language includes syntactic class
definitions that permit programmers to concisely define objects that conform to the same
class-like abstraction pattern used by the built-in objects.
4.3.2 The Strict Variant of ECMAScript
The ECMAScript Language recognizes the possibility that some users of the language may wish to
restrict their usage of some features available in the language. They might do so in the
interests of security, to avoid what they consider to be error-prone features, to get enhanced
error checking, or for other reasons of their choosing. In support of this possibility,
ECMAScript defines a strict variant of the language. The strict variant of the language excludes
some specific syntactic and semantic features of the regular ECMAScript language and modifies
the detailed semantics of some features. The strict variant also specifies additional error
conditions that must be reported by throwing error exceptions in situations that are not
specified as errors by the non-strict form of the language.
The strict variant of ECMAScript is commonly referred to as the strict mode of the
language. Strict mode selection and use of the strict mode syntax and semantics of ECMAScript is
explicitly made at the level of individual ECMAScript source text units as described in
11.2.2. Because strict mode is selected at
the level of a syntactic source text unit, strict mode only imposes restrictions that have local
effect within such a source text unit. Strict mode does not restrict or modify any aspect of the
ECMAScript semantics that must operate consistently across multiple source text units. A
complete ECMAScript program may be composed of both strict mode and non-strict mode ECMAScript source
text units. In this case, strict mode only applies when actually
executing code that is defined within a strict mode source text unit.
In order to conform to this specification, an ECMAScript implementation must implement both the
full unrestricted ECMAScript language and the strict variant of the ECMAScript language as
defined by this specification. In addition, an implementation must support the combination of
unrestricted and strict mode source text units into a single composite program.
4.4 Terms and Definitions
For the purposes of this document, the following terms and definitions apply.
4.4.1 implementation-approximated
an implementation-approximated facility
is defined in whole or in part by an external source but has a recommended, ideal behaviour in
this specification
4.4.2 implementation-defined
an implementation-defined facility is defined
in whole or in part by an external source to this specification
The value of a constructor's "prototype"
property is a prototype object that is used to implement inheritance and shared
properties.
4.4.8 prototype
object that provides shared properties for other objects
Note
When a constructor creates an object, that object
implicitly references the constructor's "prototype"
property for the purpose of resolving property references. The constructor's "prototype"
property can be referenced by the program expression
constructor.prototype, and properties added to an object's
prototype are shared, through inheritance, by all objects sharing the prototype.
Alternatively, a new object may be created with an explicitly specified prototype by
using the Object.create built-in function.
4.4.9 ordinary object
object that has the default behaviour for the essential internal methods that must be supported
by all objects
4.4.10 exotic object
object that does not have the default behaviour for one or more of the essential internal methods
There are only two Boolean values, true and false.
4.4.18 Boolean type
type consisting of the primitive values true and false
4.4.19 Boolean object
member of the Object
type that is an instance of the standard built-in Boolean constructor
Note
A Boolean object is created by using the Boolean constructor in a
new expression, supplying a Boolean value as an argument. The resulting
object has an internal slot whose value is the Boolean value. A Boolean object can be
coerced to a Boolean value.
4.4.20 String value
primitive value that is a finite ordered sequence of zero or more 16-bit unsigned
integer values
Note
A String value is a member of the String type.
Each integer
value in the sequence usually represents a single 16-bit unit of UTF-16 text. However,
ECMAScript does not place any restrictions or requirements on the values except that
they must be 16-bit unsigned integers.
4.4.21 String type
set of all possible String values
4.4.22 String object
member of the Object
type that is an instance of the standard built-in String constructor
Note
A String object is created by using the String constructor in a
new expression, supplying a String value as an argument. The resulting
object has an internal slot whose value is the String value. A String object can be
coerced to a String value by calling the String constructor as a function
(22.1.1.1).
4.4.23 Number value
primitive value corresponding to a double-precision 64-bit binary format IEEE
754-2019 value
Note
A Number value is a member of the Number type and
is a direct representation of a number.
4.4.24 Number type
set of all possible Number values including NaN (“not a number”),
+∞𝔽 (positive infinity), and -∞𝔽
(negative infinity)
4.4.25 Number object
member of the Object
type that is an instance of the standard built-in Number constructor
Note
A Number object is created by using the Number constructor in a
new expression, supplying a Number value as an argument. The resulting
object has an internal slot whose value is the Number value. A Number object can be
coerced to a Number value by calling the Number constructor as a function
(21.1.1.1).
4.4.26 Infinity
Number value that is the positive infinite Number value
4.4.27 NaN
Number value that is an IEEE 754-2019 NaN (“not a number”) value
4.4.28 BigInt value
primitive value corresponding to an arbitrary-precision integer value
4.4.29 BigInt type
set of all possible BigInt values
4.4.30 BigInt object
member of the Object
type that is an instance of the standard built-in BigInt constructor
4.4.31 Symbol value
primitive value that represents a unique, non-String Object property key
4.4.32 Symbol type
set of all possible Symbol values
4.4.33 Symbol object
member of the Object
type that is an instance of the standard built-in Symbol constructor
4.4.34 function
member of the Object
type that may be invoked as a subroutine
Note
In addition to its properties, a function contains executable code and state that
determine how it behaves when invoked. A function's code may or may not be written in
ECMAScript.
4.4.35 built-in function
built-in object that is a function
Note
Examples of built-in functions include parseInt and Math.exp. A
host or
implementation may provide additional built-in functions that are not described in this
specification.
Examples of built-in constructors include Object and
Function. A host or implementation may provide additional
built-in constructors that are not described in this
specification.
4.4.37 property
part of an object that associates a key (either a String value or a Symbol value) and a value
Note
Depending upon the form of the property the value may be represented either directly as a
data value (a primitive value, an object, or a function object) or
indirectly by a pair of accessor functions.
4.4.38 method
function that is the value of a property
Note
When a function is called as a method of an object, the object is passed to the function
as its this value.
4.4.39 built-in method
method that is a built-in function
Note
Standard built-in methods are defined in this specification. A host or implementation may provide
additional built-in methods that are not described in this specification.
4.4.40 attribute
internal value that defines some characteristic of a property
4.4.41 own property
property that is directly contained by its object
4.4.42 inherited property
property of an object that is not an own property but is a property (either own or inherited) of
the object's prototype
4.5 Organization of This Specification
The remainder of this specification is organized as follows:
Clause 5 defines the notational conventions used
throughout the specification.
Clauses 6 through 10 define the execution
environment within which ECMAScript programs operate.
Clauses 11 through 17 define the actual
ECMAScript programming language including its syntactic encoding and the execution semantics of all
language features.
Clauses 18 through 28 define the
ECMAScript standard library. They include the definitions of all of the standard objects that are
available for use by ECMAScript programs as they execute.
Clause 29
describes the memory consistency model of accesses on SharedArrayBuffer-backed memory and methods of
the Atomics object.
5 Notational Conventions
5.1 Syntactic and Lexical Grammars
5.1.1 Context-Free Grammars
A context-free grammar consists of a number of productions. Each production has
an abstract symbol called a nonterminal as its left-hand side, and a sequence
of zero or more nonterminal and terminal symbols as its right-hand side. For
each grammar, the terminal symbols are drawn from a specified alphabet.
A chain production is a production that has
exactly one nonterminal symbol on its right-hand side along with zero or more terminal symbols.
Starting from a sentence consisting of a single distinguished nonterminal, called the goal symbol, a given context-free grammar
specifies a language, namely, the (perhaps infinite) set of possible sequences of
terminal symbols that can result from repeatedly replacing any nonterminal in the sequence with
a right-hand side of a production for which the nonterminal is the left-hand side.
Input elements other than white space and comments form the terminal symbols for the syntactic
grammar for ECMAScript and are called ECMAScript tokens. These tokens are the reserved words, identifiers,
literals, and punctuators of the ECMAScript language. Moreover, line terminators, although not
considered to be tokens, also become part of the stream of input elements and guide the process
of automatic semicolon insertion (12.10). Simple
white space and single-line comments are discarded and do not appear in the stream of input
elements for the syntactic grammar. A MultiLineComment (that is, a comment of the
form /*…*/ regardless of whether it spans more than one line) is
likewise simply discarded if it contains no line terminator; but if a MultiLineComment contains one or more line
terminators, then it is replaced by a single line terminator, which becomes part of the stream
of input elements for the syntactic grammar.
A RegExp grammar for ECMAScript is given in 22.2.1. This grammar also has as its
terminal symbols the code points as defined by SourceCharacter. It defines a set of
productions, starting from the goal symbolPattern, that describe how sequences of code points
are translated into regular expression patterns.
Productions of the lexical and RegExp grammars are distinguished by having two colons “::”
as separating punctuation. The lexical and RegExp grammars share some productions.
Productions of the numeric string grammar are distinguished by having three colons “:::”
as punctuation, and are never used for parsing source text.
5.1.4 The Syntactic Grammar
The syntactic grammar for ECMAScript is given in clauses 13 through 16. This grammar has
ECMAScript tokens defined by the lexical grammar as its terminal symbols (5.1.2). It defines a set of
productions, starting from two alternative goal symbolsScript and Module, that describe how sequences of tokens form
syntactically correct independent components of ECMAScript programs.
When a stream of code points is to be parsed as an ECMAScript Script or Module, it is first converted to a stream of input
elements by repeated application of the lexical grammar; this stream of input elements is then
parsed by a single application of the syntactic grammar. The input stream is syntactically in
error if the tokens in the stream of input elements cannot be parsed as a single instance of the
goal nonterminal (Script or Module), with no tokens left over.
When a parse is successful, it constructs a parse tree, a rooted tree structure in which
each node is a Parse Node. Each Parse Node is an
instance of a symbol in the grammar; it represents a span of the source text that can
be derived from that symbol. The root node of the parse tree, representing the whole of the
source text, is an instance of the parse's goal symbol. When a Parse
Node is an instance of a nonterminal, it is also an instance of some production that has that
nonterminal as its left-hand side. Moreover, it has zero or more children, one for each
symbol on the production's right-hand side: each child is a Parse Node that is an instance of
the corresponding symbol.
New Parse Nodes are instantiated for each invocation of the parser and never reused between
parses even of identical source text. Parse Nodes are considered the same
Parse Node if and only if they represent the same span of source text, are instances
of the same grammar symbol, and resulted from the same parser invocation.
Note 1
Parsing the same String multiple times will lead to different Parse Nodes. For example,
consider:
let str = "1 + 1;";
eval(str);
eval(str);
Each call to eval converts the value of str into ECMAScript source
text and performs an independent parse that creates its own
separate tree of Parse Nodes. The trees are distinct even though each parse operates
upon a source text that was derived from the same String value.
Note 2
Parse Nodes are specification artefacts, and implementations are not
required to use an analogous data structure.
Productions of the syntactic grammar are distinguished by having just one colon “:” as
punctuation.
The syntactic grammar as presented in clauses 13 through 16 is not a complete
account of which token sequences are accepted as a correct ECMAScript Script or Module. Certain additional token sequences are also
accepted, namely, those that would be described by the grammar if only semicolons were added to
the sequence in certain places (such as before line terminator characters). Furthermore, certain
token sequences that are described by the grammar are not considered acceptable if a line
terminator character appears in certain “awkward” places.
In certain cases, in order to avoid ambiguities, the syntactic grammar uses generalized
productions that permit token sequences that do not form a valid ECMAScript Script or Module. For example, this technique is used for object
literals and object destructuring patterns. In such cases a more restrictive supplemental
grammar is provided that further restricts the acceptable token sequences. Typically,
an early
error rule will then state that, in certain contexts, "P must cover an N", where P is a
Parse Node (an instance of the generalized production) and N is a nonterminal from
the supplemental grammar. This means:
The sequence of tokens originally matched by P is parsed again using N
as the goal symbol. If N takes
grammatical parameters, then they are set to the same values used when P was
originally parsed.
If the sequence of tokens can be parsed as a single instance of N, with no tokens
left over, then:
We refer to that instance of N (a Parse Node, unique for a given
P) as "the N that is covered by
P".
All Early Error rules for N and its derived productions also apply to the
N that is covered by P.
Otherwise (if the parse fails), it is an early Syntax Error.
5.1.5 Grammar Notation
5.1.5.1 Terminal Symbols
In the ECMAScript grammars, some terminal symbols are shown in fixed-width font.
These are to appear in a source text exactly as written. All terminal symbol code points
specified in this way are to be understood as the appropriate Unicode code points from the
Basic Latin block, as opposed to any similar-looking code points from other Unicode ranges.
A code point in a terminal symbol cannot be expressed by a \UnicodeEscapeSequence.
In grammars whose terminal symbols are individual Unicode code points (i.e., the lexical,
RegExp, and numeric string grammars), a contiguous run of multiple fixed-width code points
appearing in a production is a simple shorthand for the same sequence of code points,
written as standalone terminal symbols.
In contrast, in the syntactic grammar, a contiguous run of fixed-width code points is a
single terminal symbol.
Terminal symbols come in two other forms:
In the lexical and RegExp grammars, Unicode code points without a conventional printed
representation are instead shown in the form "<ABBREV>" where "ABBREV" is a
mnemonic for the code point or set of code points. These forms are defined in Unicode Format-Control
Characters, White Space, and Line Terminators.
In the syntactic grammar, certain terminal symbols (e.g. IdentifierName and RegularExpressionLiteral) are
shown in italics, as they refer to the nonterminals of the same name in the lexical
grammar.
5.1.5.2 Nonterminal Symbols and Productions
Nonterminal symbols are shown in italic type. The definition of a nonterminal (also
called a “production”) is introduced by the name of the nonterminal being defined followed
by one or more colons. (The number of colons indicates to which grammar the production
belongs.) One or more alternative right-hand sides for the nonterminal then follow on
succeeding lines. For example, the syntactic definition:
states that the nonterminal WhileStatement represents
the token while, followed by a left parenthesis token, followed by an Expression, followed by a right
parenthesis token, followed by a Statement. The occurrences of Expression and Statement are themselves
nonterminals. As another example, the syntactic definition:
states that an ArgumentList may represent
either a single AssignmentExpression or an ArgumentList,
followed by a comma, followed by an AssignmentExpression. This definition
of ArgumentList is recursive,
that is, it is defined in terms of itself. The result is that an ArgumentList may contain any
positive number of arguments, separated by commas, where each argument expression is an
AssignmentExpression. Such recursive
definitions of nonterminals are common.
5.1.5.3 Optional Symbols
The subscripted suffix “opt”, which may appear after a terminal or nonterminal,
indicates an optional symbol. The alternative containing the optional symbol actually
specifies two right-hand sides, one that omits the optional element and one that includes
it. This means that:
so, in this example, the nonterminal ForStatement actually has
four alternative right-hand sides.
5.1.5.4 Grammatical Parameters
A production may be parameterized by a subscripted annotation of the form
“[parameters]”, which may appear as a suffix to the nonterminal symbol defined by
the production. “parameters” may be either a single name or a comma separated
list of names. A parameterized production is shorthand for a set of productions defining all
combinations of the parameter names, preceded by an underscore, appended to the
parameterized nonterminal symbol. This means that:
Prefixing a parameter name with “?” on a right-hand side nonterminal reference
makes that parameter value dependent upon the occurrence of the parameter name on the
reference to the current production's left-hand side symbol. For example:
If a right-hand side alternative is prefixed with “[+parameter]” that alternative is only
available if the named parameter was used in referencing the production's nonterminal
symbol. If a right-hand side alternative is prefixed with “[~parameter]” that alternative is
only available if the named parameter was not used in referencing the production's
nonterminal symbol. This means that:
When the words “one of” follow the colon(s) in a grammar definition, they signify that
each of the terminal symbols on the following line or lines is an alternative definition.
For example, the lexical grammar for ECMAScript contains the production:
If the phrase “[empty]” appears as the right-hand side of a production, it indicates that the
production's right-hand side contains no terminals or nonterminals.
5.1.5.7 Lookahead Restrictions
If the phrase “[lookahead = seq]” appears in the right-hand side of a production,
it indicates that the production may only be used if the token sequence seq is a
prefix of the immediately following input token sequence. Similarly, “[lookahead ∈
set]”, where set is a finite non-empty set of token sequences, indicates
that the production may only be used if some element of set is a prefix of the
immediately following token sequence. For convenience, the set can also be written as a
nonterminal, in which case it represents the set of all token sequences to which that
nonterminal could expand. It is considered an editorial error if the nonterminal could
expand to infinitely many distinct token sequences.
These conditions may be negated. “[lookahead ≠ seq]” indicates that the containing
production may only be used if seq is not a prefix of the immediately
following input token sequence, and “[lookahead ∉ set]” indicates that the
production may only be used if no element of set is a prefix of the
immediately following token sequence.
matches either the letter n followed by one or more decimal digits the first of
which is even, or a decimal digit not followed by another decimal digit.
Note that when these phrases are used in the syntactic grammar, it may not be possible to
unambiguously identify the immediately following token sequence because determining later
tokens requires knowing which lexical goal symbol to use
at later positions. As such, when these are used in the syntactic grammar, it is considered
an editorial error for a token sequence seq to appear in a lookahead restriction
(including as part of a set of sequences) if the choices of lexical goal symbols to use could change
whether or not seq would be a prefix of the resulting token sequence.
If the phrase “[no LineTerminator here]” appears in the
right-hand side of a production of the syntactic grammar, it indicates that the production
is a restricted production: it may not be used if a LineTerminator occurs in the input stream
at the indicated position. For example, the production:
indicates that the production may not be used if a LineTerminator occurs in the script between
the throw token and the Expression.
Unless the presence of a LineTerminator is forbidden by a restricted
production, any number of occurrences of LineTerminator may appear between any two
consecutive tokens in the stream of input elements without affecting the syntactic
acceptability of the script.
5.1.5.9 but not
The right-hand side of a production may specify that certain expansions are not permitted by
using the phrase “but not” and then indicating the expansions to be excluded. For
example, the production:
means that the nonterminal Identifier may be replaced by
any sequence of code points that could replace IdentifierName provided that the same
sequence of code points could not replace ReservedWord.
5.1.5.10 Descriptive Phrases
Finally, a few nonterminal symbols are described by a descriptive phrase in sans-serif type
in cases where it would be impractical to list all the alternatives:
The specification often uses a numbered list to specify steps in an algorithm. These algorithms are
used to precisely specify the required semantics of ECMAScript language constructs. The algorithms
are not intended to imply the use of any specific implementation technique. In practice, there may
be more efficient algorithms available to implement a given feature.
Algorithms may be explicitly parameterized with an ordered, comma-separated sequence of alias names
which may be used within the algorithm steps to reference the argument passed in that position.
Optional parameters are denoted with surrounding brackets ([ , name ]) and are no
different from required parameters within algorithm steps. A rest parameter may appear at the end of
a parameter list, denoted with leading ellipsis (, ...name). The rest parameter captures
all of the arguments provided following the required and optional parameters into a List. If there are no such
additional arguments, that List is empty.
Algorithm steps may be subdivided into sequential substeps. Substeps are indented and may themselves
be further divided into indented substeps. Outline numbering conventions are used to identify
substeps with the first level of substeps labelled with lowercase alphabetic characters and the
second level of substeps labelled with lowercase roman numerals. If more than three levels are
required these rules repeat with the fourth level using numeric labels. For example:
Top-level step
Substep.
Substep.
Subsubstep.
Subsubsubstep
Subsubsubsubstep
Subsubsubsubsubstep
A step or substep may be written as an “if” predicate that conditions its substeps. In this case, the
substeps are only applied if the predicate is true. If a step or substep begins with the word
“else”, it is a predicate that is the negation of the preceding “if” predicate step at the same
level.
A step may specify the iterative application of its substeps.
A step that begins with “Assert:” asserts an invariant condition
of its algorithm. Such assertions are used to make explicit algorithmic invariants that would
otherwise be implicit. Such assertions add no additional semantic requirements and hence need not be
checked by an implementation. They are used simply to clarify algorithms.
Algorithm steps may declare named aliases for any value using the form “Let x be
someValue”. These aliases are reference-like in that both x and
someValue refer to the same underlying data and modifications to either are visible to
both. Algorithm steps that want to avoid this reference-like behaviour should explicitly make a copy
of the right-hand side: “Let x be a copy of someValue” creates a shallow copy
of someValue.
Once declared, an alias may be referenced in any subsequent steps and must not be referenced from
steps prior to the alias's declaration. Aliases may be modified using the form “Set x to
someOtherValue”.
5.2.1 Abstract Operations
In order to facilitate their use in multiple parts of this specification, some algorithms, called
abstract operations, are named and written in parameterized functional
form so that they may be referenced by name from within other algorithms. Abstract operations
are typically referenced using a functional application style such as
OperationName(arg1, arg2). Some abstract operations are treated as
polymorphically dispatched methods of class-like specification abstractions. Such method-like
abstract operations are typically referenced using a method application style such as
someValue.OperationName(arg1, arg2).
5.2.2 Syntax-Directed Operations
A syntax-directed operation is a
named operation whose definition consists of algorithms, each of which is associated with one or
more productions from one of the ECMAScript grammars. A production that has multiple alternative
definitions will typically have a distinct algorithm for each alternative. When an algorithm is
associated with a grammar production, it may reference the terminal and nonterminal symbols of
the production alternative as if they were parameters of the algorithm. When used in this
manner, nonterminal symbols refer to the actual alternative definition that is matched when
parsing the source text. The source text matched by a grammar
production or Parse Node derived from it is the portion
of the source text that starts at the beginning of the first terminal that participated in the
match and ends at the end of the last terminal that participated in the match.
When an algorithm is associated with a production alternative, the alternative is typically shown
without any “[ ]” grammar annotations. Such annotations should only affect the syntactic
recognition of the alternative and have no effect on the associated semantics for the
alternative.
Syntax-directed operations are invoked with a parse node and, optionally, other parameters by
using the conventions on steps 1, 3, and 4 in the following algorithm:
Let status be SyntaxDirectedOperation of
SomeNonTerminal.
Let someParseNode be the parse of some source text.
Perform SyntaxDirectedOperation of
someParseNode.
Perform SyntaxDirectedOperation of
someParseNode with argument "value".
Unless explicitly specified otherwise, all chain productions have an
implicit definition for every operation that might be applied to that production's left-hand
side nonterminal. The implicit definition simply reapplies the same operation with the same
parameters, if any, to the chain production's sole right-hand side
nonterminal and then returns the result. For example, assume that some algorithm has a step of
the form: “Return Evaluation of Block” and that there is a production:
but the Evaluation operation does not associate an
algorithm with that production. In that case, the Evaluation operation implicitly includes an
association of the form:
The abstract operation Completion takes argument completionRecord (a Completion Record)
and returns a Completion Record.
It is used to emphasize that a Completion
Record is being returned. It performs the following steps when
called:
Similarly, prefix ! is used to indicate that the following invocation of an
abstract or syntax-directed
operation will never return an abrupt completion
and that the resulting Completion
Record's [[Value]] field should be used in
place of the return value of the operation. For example, the step:
In algorithms within abstract
operations which are declared to return a Completion Record,
and within all built-in functions, the returned value is first passed to NormalCompletion, and the result is used
instead. This rule does not apply within the Completion algorithm or when the value
being returned is clearly marked as a Completion Record in
that step; these cases are:
when the result of constructing a Completion
Record is directly returned
It is an editorial error if a Completion
Record is returned from such an abstract operation through any other
means. For example, within these abstract
operations,
Note that, through the ReturnIfAbrupt expansion, the following
example is allowed, as within the expanded steps, the result of applying Completion
is returned directly in the abrupt case and the implicit NormalCompletion
application occurs after unwrapping in the normal case.
Return ? completion.
The following example would be an editorial error because a Completion Record is
being returned without being annotated in that step.
Context-free grammars are not sufficiently powerful to express all the rules that define whether
a stream of input elements form a valid ECMAScript Script or Module that may be evaluated. In some situations
additional rules are needed that may be expressed using either ECMAScript algorithm conventions
or prose requirements. Such rules are always associated with a production of a grammar and are
called the static semantics of the production.
Static Semantic Rules have names and typically are defined using an algorithm. Named Static
Semantic Rules are associated with grammar productions and a production that has multiple
alternative definitions will typically have for each alternative a distinct algorithm for each
applicable named static semantic rule.
A special kind of static semantic rule is an Early Error
Rule. Early
error rules define early error conditions (see clause 17) that are
associated with specific grammar productions. Evaluation of most early error rules are not explicitly
invoked within the algorithms of this specification. A conforming implementation must, prior to
the first evaluation of a Script or
Module, validate all of the
early error
rules of the productions used to parse that Script or Module. If any of the early error rules are violated the
Script or Module is invalid and cannot be evaluated.
5.2.5 Mathematical Operations
This specification makes reference to these kinds of numeric values:
Mathematical values: Arbitrary real numbers, used as the default
numeric type.
In the language of this specification, numerical values are distinguished among different numeric
kinds using subscript suffixes. The subscript 𝔽 refers to Numbers, and the subscript
ℤ refers to BigInts. Numeric values without a subscript suffix refer to mathematical
values. This specification denotes most numeric values in base 10; it
also uses numeric values of the form 0x followed by digits 0-9 or A-F as base-16 values.
In general, when this specification refers to a numerical value, such as in the phrase, "the
length of y" or "the integer represented by the four hexadecimal digits ...",
without explicitly specifying a numeric kind, the phrase refers to a mathematical
value. Phrases which refer to a Number or a BigInt value are explicitly
annotated as such; for example, "the Number value for the number of code points in …"
or "the BigInt
value for …".
When the term integer is used in this
specification, it refers to a mathematical value which is in the set of
integers, unless
otherwise stated. When the term integral Number
is used in this specification, it refers to a finite Number value whose mathematical
value is in the set of integers.
Numeric operators such as +, ×, =, and ≥ refer to those operations as determined by the type of
the operands. When applied to mathematical values, the operators refer to
the usual mathematical operations. When applied to extended mathematical
values, the operators refer to the usual mathematical operations over the
extended real numbers; indeterminate forms are not defined and their use in this specification
should be considered an editorial error. When applied to Numbers, the operators refer to the
relevant operations within IEEE 754-2019. When applied to BigInts, the
operators refer to the usual mathematical operations applied to the mathematical
value of the BigInt. Numeric operators applied to mixed-type operands
(such as a Number and a mathematical value) are not defined and should
be considered an editorial error in this specification.
The mathematical function abs(x) produces the absolute value of x,
which is -x if x < 0 and otherwise is
x itself.
The mathematical function min(x1,
x2, … , xN) produces the mathematically smallest of x1 through xN. The mathematical function max(x1, x2, ..., xN)
produces the mathematically largest of x1 through
xN. The domain and range of these mathematical
functions are the extended mathematical values.
The notation “x modulo
y” (y must be finite and non-zero) computes a value k of the
same sign as y (or zero) such that abs(k) < abs(y) and
x - k = q × y for some integerq.
The phrase "the result of clamping x between
lower and upper" (where x is an extended mathematical value and
lower and upper are mathematical values such that lower
≤ upper) produces lower if x < lower, produces
upper if x > upper, and otherwise produces x.
The mathematical function floor(x) produces the largest integer (closest to +∞) that is not larger
than x.
The mathematical function truncate(x) removes the fractional part of x
by rounding towards zero, producing -floor(-x) if
x < 0 and otherwise producing floor(x).
Mathematical functions min,
max, abs, floor, and truncate are not defined for Numbers
and BigInts, and any usage of those methods that have non-mathematical value arguments
would be an editorial error in this specification.
An interval from lower bound a to upper bound
b is a possibly-infinite, possibly-empty set of numeric values of the same numeric
type. Each bound will be described as either inclusive or exclusive, but not both. There are
four kinds of intervals, as follows:
An interval
from a (inclusive) to b (inclusive), also called an inclusive interval from a to
b, includes all values x of the same numeric type such that
a ≤ x ≤ b, and no others.
An interval
from a (inclusive) to b (exclusive) includes all values x
of the same numeric type such that a ≤ x < b, and no
others.
An interval
from a (exclusive) to b (inclusive) includes all values x
of the same numeric type such that a < x ≤ b, and no
others.
An interval
from a (exclusive) to b (exclusive) includes all values x
of the same numeric type such that a < x < b, and no
others.
For example, the interval from 1 (inclusive) to 2 (exclusive) consists of
all mathematical values between 1 and 2, including
1 and not including 2. For the purpose of defining intervals, -0𝔽
<
+0𝔽, so, for example, an inclusive interval with a lower
bound of +0𝔽 includes +0𝔽 but not
-0𝔽. NaN is never included in an interval.
5.2.6 Value Notation
In this specification, ECMAScript language values are
displayed in bold. Examples include null,
true, or "hello". These are distinguished from ECMAScript source
text such as Function.prototype.apply or
let n = 42;.
5.2.7 Identity
In this specification, both specification values and ECMAScript language values are
compared for equality. When comparing for equality, values fall into one of two categories. Values without
identity are equal to other values without identity if all of their innate
characteristics are the same — characteristics such as the magnitude of an integer or the length of a
sequence. Values without identity may be manifest without prior reference by fully describing
their characteristics. In contrast, each value with identity is unique and therefore only equal to itself. Values
with identity are like values without identity but with an additional unguessable, unchangeable,
universally-unique characteristic called identity. References to existing values with
identity cannot be manifest simply by describing them, as the identity itself is indescribable;
instead, references to these values must be explicitly passed from one place to another. Some
values with identity are mutable and therefore can have their characteristics (except their
identity) changed in-place, causing all holders of the value to observe the new characteristics.
A value without identity is never equal to a value with identity.
From the perspective of this specification, the word “is” is used to compare two values for
equality, as in “If bool is true, then ...”, and the word
“contains” is used to search for a value inside lists using equality comparisons, as in "If
list contains a Recordr such that r.[[Foo]] is true,
then ...". The specification identity of values determines the result of these
comparisons and is axiomatic in this specification.
From the perspective of the ECMAScript language, language values are compared for equality using
the SameValue
abstract operation and the abstract
operations it transitively calls. The algorithms of these comparison
abstract operations
determine language identity of ECMAScript language
values.
Algorithms within this specification manipulate values each of which has an associated type. The possible
value types are exactly those defined in this clause. Types are further classified into ECMAScript language types and specification
types.
6.1 ECMAScript Language Types
An ECMAScript language type corresponds
to values that are directly manipulated by an ECMAScript programmer using the ECMAScript language.
The ECMAScript language types are Undefined, Null, Boolean, String, Symbol, Number, BigInt, and
Object. An ECMAScript language value
is a value that is characterized by an ECMAScript language type.
6.1.1 The Undefined Type
The Undefined type has exactly one value, called undefined. Any variable that
has not been assigned a value has the value undefined.
6.1.2 The Null Type
The Null type has exactly one value, called null.
6.1.3 The Boolean Type
The Boolean type represents a
logical entity having two values, called true and false.
6.1.4 The String Type
The String type is the set of all
ordered sequences of zero or more 16-bit unsigned integer values (“elements”) up to a maximum length of
253 - 1 elements. The String type is generally used to represent textual data in a
running ECMAScript program, in which case each element in the String is treated as a UTF-16 code
unit value. Each element is regarded as occupying a position within the sequence. These
positions are indexed with non-negative integers. The first element (if any) is at index 0, the
next element (if any) at index 1, and so on. The length of a String is the number of elements
(i.e., 16-bit values) within it. The empty String has length zero and therefore contains no
elements.
ECMAScript operations that do not interpret String contents apply no further semantics.
Operations that do interpret String values treat each element as a single UTF-16 code unit.
However, ECMAScript does not restrict the value of or relationships between these code units, so
operations that further interpret String contents as sequences of Unicode code points encoded in
UTF-16 must account for ill-formed subsequences. Such operations apply special treatment to
every code unit with a numeric value in the inclusive interval from 0xD800
to 0xDBFF (defined by the Unicode Standard as a leading surrogate, or more formally as a
high-surrogate code unit) and every code unit with a numeric value in
the inclusive
interval from 0xDC00 to 0xDFFF (defined as a trailing surrogate, or more formally as a
low-surrogate code unit) using the following rules:
A sequence of two code units, where the first code unit c1 is a leading
surrogate and the second code unit c2 a trailing
surrogate, is a surrogate pair and is interpreted as a code point with the value
(c1 - 0xD800) × 0x400 + (c2 - 0xDC00) + 0x10000. (See 11.1.3)
The function String.prototype.normalize (see 22.1.3.15) can be used to
explicitly normalize a String value. String.prototype.localeCompare (see 22.1.3.12) internally
normalizes String values, but no other operations implicitly normalize the strings upon which
they operate. Operation results are not language- and/or locale-sensitive unless stated
otherwise.
Note
The rationale behind this design was to keep the implementation of Strings as simple and
high-performing as possible. If ECMAScript source text is in Normalized
Form C, string literals are guaranteed to also be normalized, as long as they do not
contain any Unicode escape sequences.
In this specification, the phrase "the string-concatenation of A, B, ..." (where each
argument is a String value, a code unit, or a sequence of code units) denotes the String value
whose sequence of code units is the concatenation of the code units (in order) of each of the
arguments (in order).
The phrase "the substring of S from
inclusiveStart to exclusiveEnd" (where S is a String value or a
sequence of code units and inclusiveStart and exclusiveEnd are integers) denotes the
String value consisting of the consecutive code units of S beginning at index
inclusiveStart and ending immediately before index exclusiveEnd (which is
the empty String when inclusiveStart = exclusiveEnd). If the "to" suffix
is omitted, the length of S is used as the value of exclusiveEnd.
The phrase "the ASCII word characters"
denotes the following String value, which consists solely of every letter and number in the
Unicode Basic Latin block along with U+005F (LOW LINE): "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789_".
For historical reasons, it has significance to various algorithms.
The abstract operation StringIndexOf takes arguments string (a String),
searchValue (a String), and fromIndex (a non-negative integer) and returns a
non-negative integer or not-found. It
performs the following steps when called:
Let len be the length of string.
If searchValue is the empty String and fromIndex ≤
len, return fromIndex.
Let searchLen be the length of searchValue.
For each integeri such that
fromIndex ≤ i ≤ len - searchLen, in
ascending order, do
Let candidate be the substring of
string from i to i + searchLen.
If candidate is searchValue, return i.
Return not-found.
Note 1
If searchValue is the empty String and fromIndex ≤ the length
of string, this algorithm returns fromIndex. The empty String
is effectively found at every position within a string, including after the last
code unit.
Note 2
This algorithm always returns not-found if
fromIndex + the length of searchValue > the length of
string.
The abstract operation StringLastIndexOf takes arguments string (a String),
searchValue (a String), and fromIndex (a non-negative integer) and returns a
non-negative integer or not-found. It
performs the following steps when called:
For each integeri such that 0 ≤
i ≤ fromIndex, in descending order, do
Let candidate be the substring of
string from i to i + searchLen.
If candidate is searchValue, return i.
Return not-found.
Note
If searchValue is the empty String, this algorithm returns
fromIndex. The empty String is effectively found at every position within
a string, including after the last code unit.
6.1.5 The Symbol Type
The Symbol type is the set of all
non-String values that may be used as the key of an Object property (6.1.7).
Each possible Symbol value is unique and immutable.
Each Symbol value immutably holds an associated value called [[Description]] that is either undefined or a String
value.
6.1.5.1 Well-Known Symbols
Well-known symbols are built-in Symbol values that are explicitly referenced by algorithms of
this specification. They are typically used as the keys of properties whose values serve as
extension points of a specification algorithm. Unless otherwise specified, well-known
symbols values are shared by all realms (9.3).
Within this specification a well-known symbol is referred to using the standard intrinsic notation where the
intrinsic is one of the values listed in Table 1.
Note
Previous editions of this specification used a notation of the
form @@name, where the current edition would use %Symbol.name%. In
particular, the following names were used: @@asyncIterator, @@hasInstance,
@@isConcatSpreadable, @@iterator, @@match, @@matchAll,
@@replace, @@search, @@species, @@split, @@toPrimitive, @@toStringTag, and
@@unscopables.
Table 1: Well-known Symbols
Specification Name
[[Description]]
Value and Purpose
%Symbol.asyncIterator%
"Symbol.asyncIterator"
A method that returns the default async
iterator for an object. Called by the semantics
of the for-await-of statement.
%Symbol.hasInstance%
"Symbol.hasInstance"
A method that determines if a constructor
object recognizes an object as one of the constructor's
instances. Called by the semantics of the instanceof
operator.
%Symbol.isConcatSpreadable%
"Symbol.isConcatSpreadable"
A Boolean valued property that if true indicates that an object should
be flattened to its array elements by Array.prototype.concat.
%Symbol.iterator%
"Symbol.iterator"
A method that returns the default iterator for an
object. Called by the semantics of the for-of statement.
%Symbol.match%
"Symbol.match"
A regular expression method that matches the regular expression against
a string. Called by the String.prototype.match
method.
%Symbol.matchAll%
"Symbol.matchAll"
A regular expression method that returns an iterator that
yields matches of the regular expression against a string. Called by the
String.prototype.matchAll
method.
%Symbol.replace%
"Symbol.replace"
A regular expression method that replaces matched substrings of a
string. Called by the String.prototype.replace
method.
%Symbol.search%
"Symbol.search"
A regular expression method that returns the index within a string that
matches the regular expression. Called by the String.prototype.search
method.
%Symbol.species%
"Symbol.species"
A function valued property that is the constructor
function that is used to create derived objects.
%Symbol.split%
"Symbol.split"
A regular expression method that splits a string at the indices that
match the regular expression. Called by the String.prototype.split
method.
%Symbol.toPrimitive%
"Symbol.toPrimitive"
A method that converts an object to a corresponding primitive value.
Called by the ToPrimitive abstract
operation.
%Symbol.toStringTag%
"Symbol.toStringTag"
A String valued property that is used in the creation of the default
string description of an object. Accessed by the built-in method
Object.prototype.toString.
%Symbol.unscopables%
"Symbol.unscopables"
An object valued property whose own and inherited property names are
property names that are excluded from the with environment
bindings of the associated object.
6.1.6 Numeric Types
ECMAScript has two built-in numeric types: Number and BigInt. The following abstract operations
are defined over these numeric types. The "Result" column shows the return type, along with an
indication if it is possible for some invocations of the operation to return an abrupt completion.
Because the numeric types are in general not convertible without loss of precision or truncation,
the ECMAScript language provides no implicit conversion among these types. Programmers must
explicitly call Number and BigInt functions to convert among types
when calling a function which requires another type.
Note
The first and subsequent editions of ECMAScript have provided, for certain operators,
implicit numeric conversions that could lose precision or truncate. These legacy
implicit conversions are maintained for backward compatibility, but not provided for
BigInt in order to minimize opportunity for programmer error, and to leave open the
option of generalized value types in a future edition.
6.1.6.1 The Number Type
The Number type has exactly
18,437,736,874,454,810,627 (that is, 264 - 253
+ 3) values, representing the double-precision floating point IEEE
754-2019 binary64 values as specified in the IEEE Standard for Binary
Floating-Point Arithmetic, except that the 9,007,199,254,740,990 (that is, 253 - 2) distinct NaN values of the IEEE Standard
are represented in ECMAScript as a single special NaN value. (Note that
the NaN value is produced by the program expression NaN.) In
some implementations, external code might be able to detect a difference between various NaN
values, but such behaviour is implementation-defined; to ECMAScript
code, all NaN values are indistinguishable from each other.
Note
The bit pattern that might be observed in an ArrayBuffer (see 25.1) or a SharedArrayBuffer
(see 25.2) after a Number
value has been stored into it is not necessarily the same as the internal
representation of that Number value used by the ECMAScript implementation.
There are two other special values, called positive Infinity and
negative Infinity. For brevity, these values are also referred to for
expository purposes by the symbols +∞𝔽 and
-∞𝔽, respectively. (Note that these two infinite Number
values are produced by the program expressions +Infinity (or simply
Infinity) and -Infinity.)
The other 18,437,736,874,454,810,624 (that is, 264 -
253) values are called the finite numbers. Half of these are positive numbers and half are
negative numbers; for every finite positive Number value there is a corresponding
negative value having the same magnitude.
Note that there is both a positive zero and a negative
zero. For brevity, these values are also referred to for expository purposes
by the symbols +0𝔽 and -0𝔽,
respectively. (Note that these two different zero Number values are produced by the program
expressions +0 (or simply 0) and -0.)
The 18,437,736,874,454,810,622 (that is, 264 -
253 - 2) finite non-zero values are of two kinds:
18,428,729,675,200,069,632 (that is, 264 -
254) of them are normalized, having the form
The remaining 9,007,199,254,740,990 (that is, 253 -
2) values are denormalized, having the form
s × m × 2e
where s is 1 or -1, m is an integer in the interval from 0 (exclusive) to
252 (exclusive), and e is -1074.
Note that all the positive and negative integers whose magnitude is no greater than
253 are representable in the Number type. The integer 0 has two representations in
the Number type: +0𝔽 and -0𝔽.
A finite number has
an odd significand if it is non-zero and the integerm used to express it
(in one of the two forms shown above) is odd. Otherwise, it has an even
significand.
In this specification, the phrase “the Number value for x” where
x represents an exact real mathematical quantity (which might even be an
irrational number such as π) means a Number value chosen in the following manner. Consider
the set of all finite values of the Number type, with
-0𝔽 removed and with two additional values added to it that
are not representable in the Number type, namely 21024 (which is +1 × 253 × 2971) and -21024 (which is -1 ×
253 × 2971). Choose the member of this set that is
closest in value to x. If two values of the set are equally close, then the one
with an even significand is chosen; for this purpose, the two extra values 21024
and -21024 are considered to have even
significands. Finally, if 21024 was chosen, replace it with
+∞𝔽; if -21024
was chosen, replace it with -∞𝔽; if
+0𝔽 was chosen, replace it with
-0𝔽 if and only if x < 0; any other chosen
value is used unchanged. The result is the Number value forx.
(This procedure corresponds exactly to the behaviour of the IEEE
754-2019 roundTiesToEven mode.)
Some ECMAScript operators deal only with integers in specific ranges such as the inclusive
interval from -231 to
231 - 1 or the inclusive
interval from 0 to 216 -
1. These operators accept any value of the Number type but first convert each
such value to an integer value in the expected range. See the
descriptions of the numeric conversion operations in 7.1.
6.1.6.1.1 Number::unaryMinus ( x )
The abstract operation Number::unaryMinus takes argument x (a Number) and
returns a Number. It performs the following steps when called:
If x is NaN, return NaN.
Return the negation of x; that is, compute a Number with the same
magnitude but opposite sign.
6.1.6.1.2 Number::bitwiseNOT ( x )
The abstract operation Number::bitwiseNOT takes argument x (a Number) and
returns an integral Number. It performs the
following steps when called:
Return the bitwise complement of oldValue. The mathematical value of the
result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.3 Number::exponentiate ( base,
exponent )
The abstract operation Number::exponentiate takes arguments base (a Number)
and exponent (a Number) and returns a Number. It returns an implementation-approximated
value representing the result of raising base to the exponent
power. It performs the following steps when called:
If exponent is NaN, return NaN.
If exponent is either +0𝔽 or
-0𝔽, return 1𝔽.
If base is NaN, return NaN.
If base is +∞𝔽, then
If exponent > +0𝔽, return
+∞𝔽. Otherwise, return
+0𝔽.
If base is -∞𝔽, then
If exponent > +0𝔽, then
If exponent is an odd integral Number,
return -∞𝔽. Otherwise, return
+∞𝔽.
Else,
If exponent is an odd integral Number,
return -0𝔽. Otherwise, return
+0𝔽.
If base is +0𝔽, then
If exponent > +0𝔽, return
+0𝔽. Otherwise, return
+∞𝔽.
If base is -0𝔽, then
If exponent > +0𝔽, then
If exponent is an odd integral Number,
return -0𝔽. Otherwise, return
+0𝔽.
Else,
If exponent is an odd integral Number,
return -∞𝔽. Otherwise, return
+∞𝔽.
Assert:
base is finite and is neither
+0𝔽 nor -0𝔽.
Assert:
exponent is finite and is neither
+0𝔽 nor -0𝔽.
If base < -0𝔽 and
exponent is not an integral Number,
return NaN.
Return an implementation-approximated
Number value representing the result of raising ℝ(base) to the
ℝ(exponent) power.
Note
The result of base**exponent when
base is 1𝔽 or
-1𝔽 and exponent is
+∞𝔽 or -∞𝔽, or
when base is 1𝔽 and
exponent is NaN, differs from IEEE
754-2019. The first edition of ECMAScript specified a
result of NaN for this operation, whereas later revisions of
IEEE 754 specified 1𝔽. The historical ECMAScript
behaviour is preserved for compatibility reasons.
6.1.6.1.4 Number::multiply ( x, y )
The abstract operation Number::multiply takes arguments x (a Number) and
y (a Number) and returns a Number. It performs multiplication according to
the rules of IEEE 754-2019 binary double-precision
arithmetic, producing the product of x and y. It performs the
following steps when called:
Finite-precision multiplication is
commutative, but not always associative.
6.1.6.1.5 Number::divide ( x, y )
The abstract operation Number::divide takes arguments x (a Number) and
y (a Number) and returns a Number. It performs division according to the
rules of IEEE 754-2019 binary double-precision
arithmetic, producing the quotient of x and y where x
is the dividend and y is the divisor. It performs the following steps when
called:
If x is NaN or y is
NaN, return NaN.
If x is either +∞𝔽 or
-∞𝔽, then
If y is either +∞𝔽 or
-∞𝔽, return NaN.
If y is +0𝔽 or y
> +0𝔽, return x.
Return -x.
If y is +∞𝔽, then
If x is +0𝔽 or x
> +0𝔽, return
+0𝔽. Otherwise, return
-0𝔽.
If y is -∞𝔽, then
If x is +0𝔽 or x
> +0𝔽, return
-0𝔽. Otherwise, return
+0𝔽.
The abstract operation Number::remainder takes arguments n (a Number) and
d (a Number) and returns a Number. It yields the remainder from an implied
division of its operands where n is the dividend and d is the
divisor. It performs the following steps when called:
In C and C++, the remainder operator accepts only integral operands; in
ECMAScript, it also accepts floating-point operands.
Note 2
The result of a floating-point remainder operation as
computed by the % operator is not the same as the “remainder” operation
defined by IEEE 754-2019. The IEEE
754-2019 “remainder” operation computes the remainder from a
rounding division, not a truncating division, and so its behaviour is not analogous
to that of the usual integer remainder operator. Instead
the ECMAScript language defines % on floating-point operations to
behave in a manner analogous to that of the Java integer
remainder operator; this may be compared with the C library function fmod.
6.1.6.1.7 Number::add ( x, y )
The abstract operation Number::add takes arguments x (a Number) and
y (a Number) and returns a Number. It performs addition according to the
rules of IEEE 754-2019 binary double-precision
arithmetic, producing the sum of its arguments. It performs the following steps when
called:
Finite-precision addition is commutative,
but not always associative.
6.1.6.1.8 Number::subtract ( x, y )
The abstract operation Number::subtract takes arguments x (a Number) and
y (a Number) and returns a Number. It performs subtraction, producing the
difference of its operands; x is the minuend and y is the
subtrahend. It performs the following steps when called:
It is always the case that x - y produces the same result as
x + (-y).
6.1.6.1.9 Number::leftShift ( x, y )
The abstract operation Number::leftShift takes arguments x (a Number) and
y (a Number) and returns an integral Number. It
performs the following steps when called:
Return the result of left shifting lNum by shiftCount
bits. The mathematical value of the
result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.10 Number::signedRightShift ( x,
y )
The abstract operation Number::signedRightShift takes arguments x (a Number)
and y (a Number) and returns an integral Number. It
performs the following steps when called:
Return the result of performing a sign-extending right shift of lNum
by shiftCount bits. The most significant bit is propagated. The
mathematical value of the
result is exactly representable as a 32-bit two's complement bit string.
6.1.6.1.11 Number::unsignedRightShift ( x,
y )
The abstract operation Number::unsignedRightShift takes arguments x (a Number)
and y (a Number) and returns an integral Number. It
performs the following steps when called:
Return the result of performing a zero-filling right shift of lNum by
shiftCount bits. Vacated bits are filled with zero. The mathematical value of the
result is exactly representable as a 32-bit unsigned bit string.
6.1.6.1.12 Number::lessThan ( x, y )
The abstract operation Number::lessThan takes arguments x (a Number) and
y (a Number) and returns a Boolean or undefined. It
performs the following steps when called:
If ℝ(x)
< ℝ(y), return
true. Otherwise, return false.
6.1.6.1.13 Number::equal ( x, y )
The abstract operation Number::equal takes arguments x (a Number) and
y (a Number) and returns a Boolean. It performs the following steps when
called:
If x is NaN, return false.
If y is NaN, return false.
If x is y, return true.
If x is +0𝔽 and y is
-0𝔽, return true.
If x is -0𝔽 and y is
+0𝔽, return true.
Return false.
6.1.6.1.14 Number::sameValue ( x, y )
The abstract operation Number::sameValue takes arguments x (a Number) and
y (a Number) and returns a Boolean. It performs the following steps when
called:
If x is NaN and y is
NaN, return true.
If x is +0𝔽 and y is
-0𝔽, return false.
If x is -0𝔽 and y is
+0𝔽, return false.
If x is y, return true.
Return false.
6.1.6.1.15 Number::sameValueZero ( x,
y )
The abstract operation Number::sameValueZero takes arguments x (a Number) and
y (a Number) and returns a Boolean. It performs the following steps when
called:
If x is NaN and y is
NaN, return true.
If x is +0𝔽 and y is
-0𝔽, return true.
If x is -0𝔽 and y is
+0𝔽, return true.
If x is y, return true.
Return false.
6.1.6.1.16 NumberBitwiseOp ( op, x,
y )
The abstract operation NumberBitwiseOp takes arguments op (&,
^, or |), x (a Number), and y (a Number)
and returns an integral Number. It performs the
following steps when called:
Let result be the result of applying the bitwise inclusive OR
operation to lBits and rBits.
Return the Number value for the integer
represented by the 32-bit two's complement bit string result.
6.1.6.1.17 Number::bitwiseAND ( x, y )
The abstract operation Number::bitwiseAND takes arguments x (a Number) and
y (a Number) and returns an integral Number. It
performs the following steps when called:
The abstract operation Number::bitwiseXOR takes arguments x (a Number) and
y (a Number) and returns an integral Number. It
performs the following steps when called:
The abstract operation Number::bitwiseOR takes arguments x (a Number) and
y (a Number) and returns an integral Number. It
performs the following steps when called:
The abstract operation Number::toString takes arguments x (a Number) and
radix (an integer in the inclusive
interval from 2 to 36) and returns a String. It represents
x as a String using a positional numeral system with radix radix.
The digits used in the representation of a number using radix r are taken
from the first r code units of
"0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation
of numbers with magnitude greater than or equal to 1𝔽
never includes leading zeroes. It performs the following steps when called:
Let n, k,
and s be integers such that k ≥ 1,
radixk - 1 ≤ s <
radixk, 𝔽(s ×
radixn - k) is x, and
k is as small as possible. Note that k is the number of
digits in the representation of s using radix radix, that
s is not divisible by radix, and that the least
significant digit of s is not necessarily uniquely determined by
these criteria.
the code unit of the most significant digit of the decimal
representation of s
the code unit 0x002E (FULL STOP)
the code units of the remaining k - 1 digits of the decimal
representation of s
the code unit 0x0065 (LATIN SMALL LETTER E)
exponentSign
the code units of the decimal representation of abs(n -
1)
Note 1
The following observations may be useful as guidelines for implementations, but
are not part of the normative requirements of this Standard:
If x is any Number value other than -0𝔽, then
ToNumber(ToString(x)) is x.
The least significant digit of s is not always uniquely determined by the
requirements listed in step 5.
Note 2
For implementations that provide more accurate conversions than required by the
rules above, it is recommended that the following alternative version of step
5 be
used as a guideline:
Let n, k, and s be integers such that k ≥
1, radixk - 1 ≤ s <
radixk, 𝔽(s ×
radixn - k) is x,
and k is as small as possible. If there are multiple
possibilities for s, choose the value of s for
which s × radixn -
k is closest in value to ℝ(x). If there
are two such possible values of s, choose the one that is
even. Note that k is the number of digits in the
representation of s using radix radix and that
s is not divisible by radix.
Note 3
Implementers of ECMAScript may find useful the paper and code written by David M.
Gay for binary-to-decimal conversion of floating-point numbers:
The BigInt type represents an
integer value.
The value may be any size and is not limited to a particular bit-width. Generally, where not
otherwise noted, operations are designed to return exact mathematically-based answers. For
binary operations, BigInts act as two's complement binary strings, with negative numbers
treated as having bits set infinitely to the left.
6.1.6.2.1 BigInt::unaryMinus ( x )
The abstract operation BigInt::unaryMinus takes argument x (a BigInt) and
returns a BigInt. It performs the following steps when called:
If x = 0ℤ, return
0ℤ.
Return -x.
6.1.6.2.2 BigInt::bitwiseNOT ( x )
The abstract operation BigInt::bitwiseNOT takes argument x (a BigInt) and
returns a BigInt. It returns the one's complement of x. It performs the
following steps when called:
Return -x - 1ℤ.
6.1.6.2.3 BigInt::exponentiate ( base,
exponent )
The abstract operation BigInt::exponentiate takes arguments base (a BigInt)
and exponent (a BigInt) and returns either a normal completion
containing a BigInt or a throw
completion. It performs the following steps when called:
If exponent < 0ℤ, throw a
RangeError exception.
If base = 0ℤ and exponent =
0ℤ, return 1ℤ.
Return base raised to the power exponent.
6.1.6.2.4 BigInt::multiply ( x, y )
The abstract operation BigInt::multiply takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when
called:
Return x × y.
Note
Even if the result has a much larger bit width than the
input, the exact mathematical answer is given.
6.1.6.2.5 BigInt::divide ( x, y )
The abstract operation BigInt::divide takes arguments x (a BigInt) and
y (a BigInt) and returns either a normal completion
containing a BigInt or a throw
completion. It performs the following steps when called:
The abstract operation BigInt::remainder takes arguments n (a BigInt) and
d (a BigInt) and returns either a normal completion
containing a BigInt or a throw
completion. It performs the following steps when called:
The sign of the result is the sign of the dividend.
6.1.6.2.7 BigInt::add ( x, y )
The abstract operation BigInt::add takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when
called:
Return x + y.
6.1.6.2.8 BigInt::subtract ( x, y )
The abstract operation BigInt::subtract takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when
called:
Return x - y.
6.1.6.2.9 BigInt::leftShift ( x, y )
The abstract operation BigInt::leftShift takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when
called:
Semantics here should be equivalent to a bitwise shift,
treating the BigInt as an infinite length string of binary two's complement digits.
6.1.6.2.10 BigInt::signedRightShift ( x,
y )
The abstract operation BigInt::signedRightShift takes arguments x (a BigInt)
and y (a BigInt) and returns a BigInt. It performs the following steps when
called:
The abstract operation BigInt::unsignedRightShift takes arguments x (a BigInt)
and y (a BigInt) and returns a throw
completion. It performs the following steps when called:
Throw a TypeError exception.
6.1.6.2.12 BigInt::lessThan ( x, y )
The abstract operation BigInt::lessThan takes arguments x (a BigInt) and
y (a BigInt) and returns a Boolean. It performs the following steps when
called:
If ℝ(x)
< ℝ(y), return
true; otherwise return false.
6.1.6.2.13 BigInt::equal ( x, y )
The abstract operation BigInt::equal takes arguments x (a BigInt) and
y (a BigInt) and returns a Boolean. It performs the following steps when
called:
If ℝ(x)
= ℝ(y),
return true; otherwise return false.
6.1.6.2.14 BinaryAnd ( x, y )
The abstract operation BinaryAnd takes arguments x (0 or 1) and y
(0 or 1) and returns 0 or 1. It performs the following steps when called:
If x = 1 and y = 1, return 1.
Else, return 0.
6.1.6.2.15 BinaryOr ( x, y )
The abstract operation BinaryOr takes arguments x (0 or 1) and y (0
or 1) and returns 0 or 1. It performs the following steps when called:
If x = 1 or y = 1, return 1.
Else, return 0.
6.1.6.2.16 BinaryXor ( x, y )
The abstract operation BinaryXor takes arguments x (0 or 1) and y
(0 or 1) and returns 0 or 1. It performs the following steps when called:
If x = 1 and y = 0, return 1.
Else if x = 0 and y = 1, return 1.
Else, return 0.
6.1.6.2.17 BigIntBitwiseOp ( op, x,
y )
The abstract operation BigIntBitwiseOp takes arguments op (&,
^, or |), x (a BigInt), and y (a BigInt)
and returns a BigInt. It performs the following steps when called:
The abstract operation BigInt::bitwiseAND takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when
called:
The abstract operation BigInt::bitwiseXOR takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when
called:
The abstract operation BigInt::bitwiseOR takes arguments x (a BigInt) and
y (a BigInt) and returns a BigInt. It performs the following steps when
called:
The abstract operation BigInt::toString takes arguments x (a BigInt) and
radix (an integer in the inclusive
interval from 2 to 36) and returns a String. It represents
x as a String using a positional numeral system with radix radix.
The digits used in the representation of a BigInt using radix r are taken
from the first r code units of
"0123456789abcdefghijklmnopqrstuvwxyz" in order. The representation
of BigInts other than 0ℤ never includes leading zeroes. It
performs the following steps when called:
Return the String value consisting of the representation of x using
radix radix.
6.1.7 The Object Type
Each instance of the Object
type, also referred to simply as “an Object”, represents a collection of properties.
Each property is either a data property, or an accessor property:
A data property associates a key value
with an ECMAScript language value and a
set of Boolean attributes.
An accessor property associates a
key value with one or two accessor functions, and a set of Boolean attributes. The accessor
functions are used to store or retrieve an ECMAScript language
value that is associated with the property.
The properties of an object are uniquely identified using property keys. A property key is either a String or a Symbol. All
Strings and Symbols, including the empty String, are valid as property keys. A property name is a property key that is a String.
Property
keys are used to access properties and their values. There are two kinds
of access for properties: get and set, corresponding to value retrieval and
assignment, respectively. The properties accessible via get and set access includes both own
properties that are a direct part of an object and inherited properties which
are provided by another associated object via a property inheritance relationship. Inherited
properties may be either own or inherited properties of the associated object. Each own property
of an object must each have a key value that is distinct from the key values of the other own
properties of that object.
All objects are logically collections of properties, but there are multiple forms of objects that
differ in their semantics for accessing and manipulating their properties. Please see 6.1.7.2 for
definitions of the multiple forms of objects.
In addition, some objects are callable; these are referred to as functions or function
objects and are described further below. All functions in ECMAScript are
members of the Object type.
6.1.7.1 Property Attributes
Attributes are used in this specification to define and explain the state of Object
properties as described in Table 3. Unless
specified explicitly, the initial value of each attribute is its Default Value.
If the value is an Object it must be a
function object. The
function's [[Call]] internal method (Table
5) is called with an empty arguments list to
retrieve the property value each time a get access of the property is
performed.
If the value is an Object it must be a
function object. The
function's [[Call]] internal method (Table
5) is called with an arguments list containing
the assigned value as its sole argument each time a set access of the
property is performed. The effect of a property's [[Set]] internal method may, but is not required
to, have an effect on the value returned by subsequent calls to the
property's [[Get]] internal method.
If false, attempts to delete the property, change it
from a data property to an
accessor property or from
an accessor property to a
data property, or make
any changes to its attributes (other than replacing an existing [[Value]] or setting [[Writable]] to false) will
fail.
6.1.7.2 Object Internal Methods and Internal Slots
The actual semantics of objects, in ECMAScript, are specified via algorithms called
internal methods. Each object in an ECMAScript engine is associated with a set of
internal methods that defines its runtime behaviour. These internal methods are not part of
the ECMAScript language. They are defined by this specification purely for expository
purposes. However, each object within an implementation of ECMAScript must behave as
specified by the internal methods associated with it. The exact manner in which this is
accomplished is determined by the implementation.
Internal method names are polymorphic. This means that different object values may perform
different algorithms when a common internal method name is invoked upon them. That actual
object upon which an internal method is invoked is the “target” of the invocation. If, at
runtime, the implementation of an algorithm attempts to use an internal method of an object
that the object does not support, a TypeError exception is thrown.
Internal slots correspond to internal state that is associated with objects and used by
various ECMAScript specification algorithms. Internal slots are not object properties and
they are not inherited. Depending upon the specific internal slot specification, such state
may consist of values of any ECMAScript language
type or of specific ECMAScript specification type values. Unless
explicitly specified otherwise, internal slots are allocated as part of the process of
creating an object and may not be dynamically added to an object. Unless specified
otherwise, the initial value of an internal slot is the value undefined.
Various algorithms within this specification create objects that have internal slots.
However, the ECMAScript language provides no direct way to associate internal slots with an
object.
All objects have an internal slot named [[PrivateElements]], which
is a List of PrivateElements. This
List represents the
values of the private fields, methods, and accessors for the object. Initially, it is an
empty List.
Internal methods and internal slots are identified within this specification using names
enclosed in double square brackets [[ ]].
Table 4 summarizes the
essential internal methods used by this specification that are applicable to all
objects created or manipulated by ECMAScript code. Every object must have algorithms for all
of the essential internal methods. However, all objects do not necessarily use the same
algorithms for those methods.
An ordinary object
is an object that satisfies all of the following criteria:
For the internal methods listed in Table 4,
the object uses those defined in 10.1.
If the object has a [[Call]] internal method, it uses either
the one defined in 10.2.1
or the one defined in 10.3.1.
If the object has a [[Construct]] internal method, it uses
either the one defined in 10.2.2
or the one defined in 10.3.2.
An exotic object is an
object that is not an ordinary object.
This specification recognizes different kinds of exotic objects by those objects'
internal methods. An object that is behaviourally equivalent to a particular kind of
exotic
object (such as an Array exotic object or a
bound function exotic object),
but does not have the same collection of internal methods specified for that kind, is not
recognized as that kind of exotic object.
The “Signature” column of Table 4 and other similar
tables describes the invocation pattern for each internal method. The invocation pattern
always includes a parenthesized list of descriptive parameter names. If a parameter name is
the same as an ECMAScript type name then the name describes the required type of the
parameter value. If an internal method explicitly returns a value, its parameter list is
followed by the symbol “→” and the type name of the returned value. The type names used in
signatures refer to the types defined in clause 6 augmented by the
following additional names. “any” means the value may be any ECMAScript language type.
In addition to its parameters, an internal method always has access to the object that is the
target of the method invocation.
Determine the object that provides inherited properties for this object.
A null value indicates that there are no inherited
properties.
[[SetPrototypeOf]]
(Object | Null) → Boolean
Associate this object with another object that provides inherited
properties. Passing null indicates that there are no
inherited properties. Returns true indicating that
the operation was completed successfully or false
indicating that the operation was not successful.
[[IsExtensible]]
( ) → Boolean
Determine whether it is permitted to add additional properties to this
object.
[[PreventExtensions]]
( ) → Boolean
Control whether new properties may be added to this object. Returns
true if the operation was successful or
false if the operation was unsuccessful.
Return a Property
Descriptor for the own property of this object
whose key is propertyKey, or undefined if
no such property exists.
[[DefineOwnProperty]]
(propertyKey, PropertyDescriptor) → Boolean
Create or alter the own property, whose key is propertyKey,
to have the state described by PropertyDescriptor. Return
true if that property was successfully
created/updated or false if the property could not be
created or updated.
[[HasProperty]]
(propertyKey) → Boolean
Return a Boolean value indicating whether this object already has either
an own or inherited property whose key is propertyKey.
[[Get]]
(propertyKey, Receiver) →any
Return the value of the property whose key is propertyKey
from this object. If any ECMAScript code must be executed to retrieve
the property value, Receiver is used as the
this value when evaluating the code.
[[Set]]
(propertyKey, value, Receiver) →
Boolean
Set the value of the property whose key is propertyKey to
value. If any ECMAScript code must be executed to set the
property value, Receiver is used as the
this value when evaluating the code. Returns
true if the property value was set or
false if it could not be set.
[[Delete]]
(propertyKey) → Boolean
Remove the own property whose key is propertyKey from this
object. Return false if the property was not deleted
and is still present. Return true if the property was
deleted or is not present.
Return a List
whose elements are all of the own property keys
for the object.
Table
5 summarizes additional essential internal methods that are supported
by objects that may be called as functions. A function object is an object that
supports the [[Call]] internal method. A constructor is an object that supports the
[[Construct]] internal method. Every object that supports [[Construct]] must support [[Call]]; that
is, every constructor must be a function
object. Therefore, a constructor may also be referred to as a
constructor function or constructorfunction
object.
Table 5: Additional Essential Internal Methods of Function Objects
Executes code associated with this object. Invoked via a function call
expression. The arguments to the internal method are a
this value and a List
whose elements are the arguments passed to the function by a call
expression. Objects that implement this internal method are
callable.
Creates an object. Invoked via the new operator or a
super call. The first argument to the internal method is a
List
whose elements are the arguments of the constructor
invocation or the super call. The second argument is the
object to which the new operator was initially applied.
Objects that implement this internal method are called constructors. A
function object is not
necessarily a constructor and such
non-constructorfunction objects do not
have a [[Construct]] internal method.
The semantics of the essential internal methods for ordinary objects and standard
exotic
objects are specified in clause 10. If any
specified use of an internal method of an exotic object is not supported by an
implementation, that usage must throw a TypeError exception when
attempted.
6.1.7.3 Invariants of the Essential Internal Methods
The Internal Methods of Objects of an ECMAScript engine must conform to the list of
invariants specified below. Ordinary ECMAScript Objects as well as all standard exotic
objects in this specification maintain these invariants. ECMAScript
Proxy objects maintain these invariants by means of runtime checks on the result of traps
invoked on the [[ProxyHandler]] object.
Any implementation provided exotic objects must also maintain these
invariants for those objects. Violation of these invariants may cause ECMAScript code to
have unpredictable behaviour and create security issues. However, violation of these
invariants must never compromise the memory safety of an implementation.
An implementation must not allow these invariants to be circumvented in any manner such as by
providing alternative interfaces that implement the functionality of the essential internal
methods without enforcing their invariants.
Definitions:
The target of an internal method is the object upon which the internal method
is called.
A target is non-extensible if it has been observed to return
false from its [[IsExtensible]] internal
method, or true from its [[PreventExtensions]] internal method.
A non-existent property is a property that does not exist as an own property on
a non-extensible target.
All references to SameValue are according to the
definition of the SameValue algorithm.
Return value:
The value returned by any internal method must be a Completion Record
with either:
[[Type]] = normal, [[Target]] = empty, and [[Value]] = a value of the "normal return type" shown below for
that internal method, or
If target is non-extensible, and [[GetPrototypeOf]] returns a
value V, then any future calls to [[GetPrototypeOf]]
should return the SameValue as V.
Note 2
An object's prototype chain should have finite length (that is, starting from any
object, recursively applying the [[GetPrototypeOf]]
internal method to its result should eventually lead to the value
null). However, this requirement is not enforceable as an object
level invariant if the prototype chain includes any exotic objects that do
not use the ordinary object definition of [[GetPrototypeOf]]. Such a circular prototype chain may
result in infinite loops when accessing object properties.
[[SetPrototypeOf]] ( V )
The normal return type is Boolean.
If target is non-extensible, [[SetPrototypeOf]] must return
false, unless V is the SameValue as the target's
observed [[GetPrototypeOf]] value.
[[IsExtensible]] ( )
The normal return type is Boolean.
If [[IsExtensible]] returns false, all
future calls to [[IsExtensible]] on the target must return
false.
[[PreventExtensions]] ( )
The normal return type is Boolean.
If [[PreventExtensions]] returns true, all
future calls to [[IsExtensible]] on the target must return
false and the target is now considered non-extensible.
If P is described as a non-configurable, non-writable own data
property, all future calls to [[GetOwnProperty]] ( P ) must return Property
Descriptor whose [[Value]] is SameValue as
P's [[Value]] attribute.
If P's attributes other than [[Writable]] and [[Value]] may change over time, or if the property might be
deleted, then P's [[Configurable]] attribute must be
true.
If the [[Writable]] attribute may change from
false to true, then the [[Configurable]] attribute must be true.
If the target is non-extensible and P is non-existent, then all future calls
to [[GetOwnProperty]] (P) on the target must
describe P as non-existent (i.e. [[GetOwnProperty]]
(P) must return undefined).
Note 3
As a consequence of the third invariant, if a property is described as a data
property and it may return different values over time, then
either or both of the [[Writable]] and [[Configurable]] attributes must be true
even if no mechanism to change the value is exposed via the other essential internal
methods.
[[DefineOwnProperty]] ( P, Desc )
The normal return type is Boolean.
[[DefineOwnProperty]] must return false if
P has previously been observed as a non-configurable own property of the
target, unless either:
All attributes of Desc are the SameValue as
P's attributes.
[[DefineOwnProperty]] (P, Desc) must
return false if target is non-extensible and P is a
non-existent own property. That is, a non-extensible target object cannot be extended
with new properties.
[[HasProperty]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor
property of the target, [[HasProperty]]
must return true.
If P was previously observed as a non-configurable, non-writable own
data
property of the target with value V, then [[Get]] must return the SameValue as V.
If P was previously observed as a non-configurable own accessor
property of the target whose [[Get]]
attribute is undefined, the [[Get]]
operation must return undefined.
[[Set]] ( P, V, Receiver )
The normal return type is Boolean.
If P was previously observed as a non-configurable, non-writable own
data
property of the target, then [[Set]]
must return false unless V is the SameValue as
P's [[Value]] attribute.
If P was previously observed as a non-configurable own accessor
property of the target whose [[Set]]
attribute is undefined, the [[Set]]
operation must return false.
[[Delete]] ( P )
The normal return type is Boolean.
If P was previously observed as a non-configurable own data or accessor
property of the target, [[Delete]] must
return false.
The returned List must contain
at least the keys of all non-configurable own properties that have previously been
observed.
If the target is non-extensible, the returned List must contain
only the keys of all own properties of the target that are observable using [[GetOwnProperty]].
The target must also have a [[Call]] internal method.
6.1.7.4 Well-Known Intrinsic Objects
Well-known intrinsics are built-in objects that are explicitly referenced by the algorithms
of this specification and which usually have realm-specific identities. Unless otherwise specified
each intrinsic object actually corresponds to a set of similar objects, one per realm.
Within this specification a reference such as %name% means the intrinsic object, associated
with the current realm, corresponding to the name. A reference such as
%name.a.b% means, as if the "b" property of the value of the
"a" property of the intrinsic object %name% was accessed prior to any
ECMAScript code being evaluated. Determination of the current realm and its intrinsics is described in
9.4. The well-known intrinsics are
listed in Table 6.
A specification type corresponds to meta-values that are used within algorithms to describe the
semantics of ECMAScript language constructs and ECMAScript language
types. The specification types include Reference Record, List, Completion Record, Property Descriptor,
Environment Record, Abstract
Closure, and Data Block. Specification type values are
specification artefacts that do not necessarily correspond to any specific entity within an
ECMAScript implementation. Specification type values may be used to describe intermediate results of
ECMAScript expression evaluation but such values cannot be stored as properties of objects or values
of ECMAScript language variables.
6.2.1 The Enum Specification Type
Enums are values which are internal to the
specification and not directly observable from ECMAScript code. Enums are denoted using a
sans-serif typeface. For instance, a Completion Record's [[Type]] field takes on values like normal,
return, or throw. Enums have no characteristics
other than their name. The name of an enum serves no purpose other than to distinguish it from
other enums, and implies nothing about its usage or meaning in context.
6.2.2 The List and Record Specification Types
The List type is used to explain the evaluation of
argument lists (see 13.3.8) in new expressions, in
function calls, and in other algorithms where a simple ordered list of values is needed. Values
of the List type are simply ordered sequences of list elements containing the individual values.
These sequences may be of any length. The elements of a list may be randomly accessed using
0-origin indices. For notational convenience an array-like syntax can be used to access List
elements. For example, arguments[2] is shorthand for saying the 3rd
element of the List arguments.
When an algorithm iterates over the elements of a List without specifying an order, the order
used is the order of the elements in the List.
For notational convenience within this specification, a literal syntax can be used to express a
new List value. For example, « 1, 2 » defines a List value that has two elements each of which
is initialized to a specific value. A new empty List can be expressed as « ».
In this specification, the phrase "the list-concatenation of A, B, ..." (where each
argument is a possibly empty List) denotes a new List value whose elements are the concatenation
of the elements (in order) of each of the arguments (in order).
As applied to a List of Strings, the phrase "sorted according to lexicographic code unit order" means
sorting by the numeric value of each code unit up to the length of the shorter string, and
sorting the shorter string before the longer string if all are equal, as described in the
abstract operation IsLessThan.
The Record type is used to describe data aggregations
within the algorithms of this specification. A Record type value consists of one or more named
fields. The value of each field is an ECMAScript language
value or specification value. Field names are always enclosed in double
brackets, for example [[Value]].
For notational convenience within this specification, an object literal-like syntax can be used
to express a Record value. For example, { [[Field1]]: 42, [[Field2]]: false, [[Field3]]: empty } defines a Record value that
has three fields, each of which is initialized to a specific value. Field name order is not
significant. Any fields that are not explicitly listed are considered to be absent.
In specification text and algorithms, dot notation may be used to refer to a specific field of a
Record value. For example, if R is the record shown in the previous paragraph then R.[[Field2]] is shorthand for “the field of R named [[Field2]]”.
Schema for commonly used Record field combinations may be named, and that name may be used as a
prefix to a literal Record value to identify the specific kind of aggregations that is being
described. For example: PropertyDescriptor { [[Value]]: 42, [[Writable]]: false, [[Configurable]]: true }.
6.2.3 The Set and Relation Specification Types
The Set type is used to explain a collection of unordered elements for use in the
memory
model. It is distinct from the ECMAScript collection type of the same
name. To disambiguate, instances of the ECMAScript collection are consistently referred to as
"Set objects" within this specification. Values of the Set type are simple collections of
elements, where no element appears more than once. Elements may be added to and removed from
Sets. Sets may be unioned, intersected, or subtracted from each other.
The Relation type is used to explain constraints on
Sets. Values of the Relation type are Sets of ordered pairs of values from its value domain. For
example, a Relation on events is a set of ordered pairs of events. For a Relation R
and two values a and b in the value domain of R, aRb is shorthand for saying the ordered pair (a, b)
is a member of R. A Relation is the least
Relation with respect to some conditions when it is the smallest Relation that
satisfies those conditions.
A strict partial order is a Relation
value R that satisfies the following.
For all a, b, and c in R's domain:
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 1
The two properties above are called irreflexivity and transitivity, respectively.
A strict total order is a Relation value
R that satisfies the following.
For all a, b, and c in R's domain:
a is b or aRb or
bRa, and
It is not the case that aRa, and
If aRb and bRc, then aRc.
Note 2
The three properties above are called totality, irreflexivity, and transitivity,
respectively.
6.2.4 The Completion Record Specification Type
The Completion Record specification type
is used to explain the runtime propagation of values and control flow such as the behaviour of
statements (break, continue, return and
throw) that perform nonlocal transfers of control.
Completion Records have the fields defined in Table 7.
The following shorthand terms are sometimes used to refer to Completion Records.
normal completion refers to any
Completion Record with a [[Type]] value of
normal.
break completion refers to any
Completion Record with a [[Type]] value of
break.
continue completion refers to any
Completion Record with a [[Type]] value of
continue.
return completion refers to any
Completion Record with a [[Type]] value of
return.
throw completion refers to any
Completion Record with a [[Type]] value of
throw.
abrupt completion refers to any
Completion Record with a [[Type]] value other than
normal.
a normal completion
containing some type of value refers to a normal completion that has a value of
that type in its [[Value]] field.
Callable objects that are defined in this specification only return a normal completion or a
throw completion. Returning any other kind of Completion Record is considered an editorial
error.
Implementation-defined callable objects
must return either a normal completion or a throw completion.
6.2.4.1 NormalCompletion ( value )
The abstract operation NormalCompletion takes argument value (any value except a
Completion Record)
and returns a normal completion.
It performs the following steps when called:
The abstract operation ThrowCompletion takes argument value (an ECMAScript language value) and
returns a throw completion. It
performs the following steps when called:
The abstract operation ReturnCompletion takes argument value (an ECMAScript language value) and
returns a return completion.
It performs the following steps when called:
The abstract operation UpdateEmpty takes arguments completionRecord (a Completion Record)
and value (any value except a Completion Record)
and returns a Completion Record.
It performs the following steps when called:
The Reference Record type is used to
explain the behaviour of such operators as delete, typeof, the
assignment operators, the superkeyword and other
language features. For example, the left-hand operand of an assignment is expected to produce a
Reference Record.
A Reference Record is a resolved name or (possibly not-yet-resolved) property binding; its fields
are defined by Table 8.
If not empty, the Reference
Record represents a property binding that was
expressed using the superkeyword; it
is called a Super Reference Record and its
[[Base]] value will never be an Environment Record.
In that case, the [[ThisValue]] field holds the
this value at the time the Reference
Record was created.
The following abstract operations
are used in this specification to operate upon Reference Records:
6.2.5.1 IsPropertyReference ( V )
The abstract operation IsPropertyReference takes argument V (a Reference Record) and
returns a Boolean. It performs the following steps when called:
If V.[[Base]] is
unresolvable, return false.
If V.[[Base]] is an Environment Record, return
false; otherwise return true.
6.2.5.2 IsUnresolvableReference ( V )
The abstract operation IsUnresolvableReference takes argument V (a Reference Record) and
returns a Boolean. It performs the following steps when called:
If V.[[Base]] is
unresolvable, return true; otherwise
return false.
6.2.5.3 IsSuperReference ( V )
The abstract operation IsSuperReference takes argument V (a Reference Record) and
returns a Boolean. It performs the following steps when called:
If V.[[ThisValue]] is not
empty, return true; otherwise return
false.
6.2.5.4 IsPrivateReference ( V )
The abstract operation IsPrivateReference takes argument V (a Reference Record) and
returns a Boolean. It performs the following steps when called:
If V.[[ReferencedName]] is a Private
Name, return true; otherwise return
false.
Return ? base.GetBindingValue(V.[[ReferencedName]], V.[[Strict]]) (see 9.1).
Note
The object that may be created in step 3.a is not
accessible outside of the above abstract operation and the ordinary
object[[Get]] internal method. An
implementation might choose to avoid the actual creation of the object.
Return ? base.SetMutableBinding(V.[[ReferencedName]], W, V.[[Strict]]) (see 9.1).
Note
The object that may be created in step 3.a is not
accessible outside of the above abstract operation and the ordinary
object[[Set]] internal method. An
implementation might choose to avoid the actual creation of that object.
The abstract operation MakePrivateReference takes arguments baseValue (an
ECMAScript language value) and
privateIdentifier (a String) and returns a Reference Record. It
performs the following steps when called:
Return the Reference
Record { [[Base]]:
baseValue, [[ReferencedName]]:
privateName, [[Strict]]:
true, [[ThisValue]]:
empty }.
6.2.6 The Property Descriptor Specification Type
The Property Descriptor type is used to
explain the manipulation and reification of Object property attributes. A Property Descriptor is
a Record with zero or more
fields, where each field's name is an attribute name and its value is a corresponding attribute
value as specified in 6.1.7.1. The schema name used within this
specification to tag literal descriptions of Property Descriptor records is
“PropertyDescriptor”.
Property Descriptor values may be further classified as data Property Descriptors and accessor
Property Descriptors based upon the existence or use of certain fields. A data Property
Descriptor is one that includes any fields named either [[Value]] or
[[Writable]]. An accessor Property Descriptor is one that includes any
fields named either [[Get]] or [[Set]]. Any
Property Descriptor may have fields named [[Enumerable]] and [[Configurable]]. A Property Descriptor value may not be both a data
Property Descriptor and an accessor Property Descriptor; however, it may be neither (in which
case it is a generic Property Descriptor). A fully populated Property
Descriptor is one that is either an accessor Property Descriptor or a data Property
Descriptor and that has all of the corresponding fields defined in Table 3.
The following abstract operations
are used in this specification to operate upon Property Descriptor values:
6.2.6.1 IsAccessorDescriptor ( Desc )
The abstract operation IsAccessorDescriptor takes argument Desc (a Property
Descriptor) and returns a Boolean. It performs the following steps
when called:
If Desc has a [[Get]] field, return
true.
If Desc has a [[Set]] field, return
true.
Return false.
6.2.6.2 IsDataDescriptor ( Desc )
The abstract operation IsDataDescriptor takes argument Desc (a Property
Descriptor) and returns a Boolean. It performs the following steps
when called:
If Desc has a [[Value]] field, return
true.
If Desc has a [[Writable]] field, return
true.
Return false.
6.2.6.3 IsGenericDescriptor ( Desc )
The abstract operation IsGenericDescriptor takes argument Desc (a Property
Descriptor) and returns a Boolean. It performs the following steps
when called:
The abstract operation FromPropertyDescriptor takes argument Desc (a Property
Descriptor or undefined) and returns an Object or
undefined. It performs the following steps when called:
If IsCallable(setter)
is false and setter is not
undefined, throw a TypeError
exception.
Set desc.[[Set]] to setter.
If desc has a [[Get]] field or desc
has a [[Set]] field, then
If desc has a [[Value]] field or
desc has a [[Writable]] field, throw a
TypeError exception.
Return desc.
6.2.6.6 CompletePropertyDescriptor ( Desc )
The abstract operation CompletePropertyDescriptor takes argument Desc (a Property
Descriptor) and returns unused. It performs
the following steps when called:
Let like be the Record { [[Value]]: undefined, [[Writable]]: false, [[Get]]: undefined, [[Set]]: undefined, [[Enumerable]]: false, [[Configurable]]: false }.
If Desc does not have a [[Value]] field,
set Desc.[[Value]] to
like.[[Value]].
If Desc does not have a [[Writable]]
field, set Desc.[[Writable]] to
like.[[Writable]].
Else,
If Desc does not have a [[Get]] field,
set Desc.[[Get]] to like.[[Get]].
If Desc does not have a [[Set]] field,
set Desc.[[Set]] to like.[[Set]].
If Desc does not have an [[Enumerable]] field,
set Desc.[[Enumerable]] to like.[[Enumerable]].
If Desc does not have a [[Configurable]] field,
set Desc.[[Configurable]] to
like.[[Configurable]].
Return unused.
6.2.7 The Environment Record Specification Type
The Environment Record type is used to
explain the behaviour of name resolution in nested functions and blocks. This type and the
operations upon it are defined in 9.1.
6.2.8 The Abstract Closure Specification Type
The Abstract Closure specification type is
used to refer to algorithm steps together with a collection of values. Abstract Closures are
meta-values and are invoked using function application style such as
closure(arg1, arg2). Like abstract
operations, invocations perform the algorithm steps described by the
Abstract Closure.
In algorithm steps that create an Abstract Closure, values are captured with the verb "capture"
followed by a list of aliases. When an Abstract Closure is created, it captures the value that
is associated with each alias at that time. In steps that specify the algorithm to be performed
when an Abstract Closure is called, each captured value is referred to by the alias that was
used to capture the value.
The Data Block specification type is used to
describe a distinct and mutable sequence of byte-sized (8 bit) numeric values. A byte value is an integer in the inclusive
interval from 0 to 255. A Data Block value is created with a fixed number
of bytes that each have the initial value 0.
For notational convenience within this specification, an array-like syntax can be used to access
the individual bytes of a Data Block value. This notation presents a Data Block value as a
0-based integer-indexed sequence of bytes. For example, if
db is a 5 byte Data Block value then db[2] can be used to access its
3rd byte.
A data block that resides in memory that can be referenced from multiple agents concurrently is designated a Shared Data Block. A Shared Data Block has
an identity (for the purposes of equality testing Shared Data Block values) that is
address-free: it is tied not to the virtual addresses the block is mapped to in any
process, but to the set of locations in memory that the block represents. Two data blocks are
equal only if the sets of the locations they contain are equal; otherwise, they are not equal
and the intersection of the sets of locations they contain is empty. Finally, Shared Data Blocks
can be distinguished from Data Blocks.
Let db be a new Shared Data Block value consisting of
size bytes. If it is impossible to create such a Shared Data
Block, throw a RangeError exception.
The abstract operation CopyDataBlockBytes takes arguments toBlock (a Data
Block or a Shared Data Block), toIndex (a
non-negative integer), fromBlock (a Data
Block or a Shared Data Block), fromIndex (a
non-negative integer), and count (a non-negative
integer) and
returns unused. It performs the following steps when called:
Assert:
fromBlock and toBlock are distinct values.
Let bytes be a List
whose sole element is a nondeterministically chosen byte value.
NOTE: In implementations, bytes is the result of a
non-atomic read instruction on the underlying hardware. The
nondeterminism is a semantic prescription of the memory model to
describe observable behaviour of hardware with weak consistency.
Let readEvent be ReadSharedMemory
{ [[Order]]:
unordered, [[NoTear]]: true, [[Block]]: fromBlock, [[ByteIndex]]: fromIndex, [[ElementSize]]: 1 }.
Append readEvent to eventsRecord.[[EventList]].
Append Chosen Value
Record { [[Event]]:
readEvent, [[ChosenValue]]:
bytes } to execution.[[ChosenValues]].
The PrivateElement type is a Record used
in the specification of private class fields, methods, and accessors. Although Property Descriptors
are not used for private elements, private fields behave similarly to non-configurable,
non-enumerable, writable data properties, private methods behave similarly
to non-configurable, non-enumerable, non-writable data properties, and private
accessors behave similarly to non-configurable, non-enumerable accessor properties.
Values of the PrivateElement type are Record
values whose fields are defined by Table 9. Such values are
referred to as PrivateElements.
6.2.11 The ClassFieldDefinition Record Specification Type
The ClassFieldDefinition type is a Record used
in the specification of class fields.
Values of the ClassFieldDefinition type are Record values whose fields
are defined by Table 10. Such values are
referred to as ClassFieldDefinition
Records.
The Private Name specification type is used to
describe a globally unique value (one which differs from any other Private Name, even if they
are otherwise indistinguishable) which represents the key of a private class element (field,
method, or accessor). Each Private Name has an associated immutable [[Description]] which is a String value. A
Private Name may be installed on any ECMAScript object with PrivateFieldAdd or PrivateMethodOrAccessorAdd, and
then read or written using PrivateGet and PrivateSet.
6.2.13 The ClassStaticBlockDefinition Record Specification Type
A ClassStaticBlockDefinition
Record is a Record value used to
encapsulate the executable code for a class static initialization block.
ClassStaticBlockDefinition Records have the fields listed in Table 11.
The function object to be called
during static initialization of a class.
7 Abstract Operations
These operations are not a part of the ECMAScript language; they are defined here solely to aid the
specification of the semantics of the ECMAScript language. Other, more specialized abstract operations are
defined throughout this specification.
7.1 Type Conversion
The ECMAScript language implicitly performs automatic type conversion as needed. To clarify the
semantics of certain constructs it is useful to define a set of conversion abstract operations.
The conversion abstract operations are
polymorphic; they can accept a value of any ECMAScript language type.
But no other specification types are used with these operations.
The BigInt type has no implicit
conversions in the ECMAScript language; programmers must call BigInt explicitly to convert values
from other types.
7.1.1 ToPrimitive ( input [ , preferredType ]
)
The abstract operation ToPrimitive takes argument input (an ECMAScript language value) and
optional argument preferredType (string or
number) and returns either a normal completion
containing an ECMAScript language
value or a throw
completion. It converts its input argument to a non-Object
type. If an object is capable of converting to more than one primitive
type, it may use the optional hint preferredType to favour that type. It performs the
following steps when called:
When ToPrimitive is called without a hint, then it generally behaves as if the hint were
number. However, objects may over-ride this behaviour by defining
a %Symbol.toPrimitive% method. Of
the objects defined in this specification only Dates (see 21.4.4.45) and
Symbol objects (see 20.4.3.5)
over-ride the default ToPrimitive behaviour. Dates treat the absence of a hint as if the
hint were string.
The abstract operation ToBoolean takes argument argument (an ECMAScript language value) and
returns a Boolean. It converts argument to a value of type Boolean. It performs the
following steps when called:
The abstract operation ToNumeric takes argument value (an ECMAScript language value) and
returns either a normal completion
containing either a Number or a BigInt, or a throw completion. It
returns value converted to a Number or a BigInt. It performs the following steps when
called:
The abstract operation RoundMVResult takes argument n (a mathematical
value) and returns a Number. It converts n to a Number
in an implementation-defined manner. For
the purposes of this abstract operation, a digit is significant if it is not zero or
there is a non-zero digit to its left and there is a non-zero digit to its right. For
the purposes of this abstract operation, "the mathematical value
denoted by" a representation of a mathematical value is the inverse of
"the decimal representation of" a mathematical value. It performs the
following steps when called:
If the decimal representation of n has 20 or fewer significant
digits, return 𝔽(n).
Let option1 be the mathematical
value denoted by the result of replacing each significant
digit in the decimal representation of n after the 20th with a 0
digit.
Let option2 be the mathematical
value denoted by the result of replacing each significant
digit in the decimal representation of n after the 20th with a 0
digit and then incrementing it at the 20th position (with carrying as
necessary).
The abstract operation ToIntegerOrInfinity takes argument argument (an ECMAScript language value) and
returns either a normal completion
containing either an integer, +∞, or -∞, or a throw completion. It
converts argument to an integer representing its Number value with fractional
part truncated, or to +∞ or -∞ when that Number value is infinite. It performs the following
steps when called:
𝔽(ToIntegerOrInfinity(x)) never returns
-0𝔽 for any value of x. The truncation of the
fractional part is performed after converting x to a mathematical
value.
Step 5 is the only difference between
ToUint32 and ToInt32.
The ToUint32 abstract operation is idempotent: if applied to a result that it
produced, the second application leaves that value unchanged.
ToUint32(ToInt32(x)) is the same value
as ToUint32(x) for all values of x. (It is to preserve this
latter property that +∞𝔽 and
-∞𝔽 are mapped to
+0𝔽.)
If f is even, return 𝔽(f). Otherwise, return 𝔽(f + 1).
Note
Unlike most other ECMAScript integer conversion operations, ToUint8Clamp
rounds rather than truncates non-integral values. It also uses “round half to even”
tie-breaking, which differs from the “round half up” tie-breaking of Math.round.
7.1.13 ToBigInt ( argument )
The abstract operation ToBigInt takes argument argument (an ECMAScript language value) and
returns either a normal completion
containing a BigInt or a throw completion. It
converts argument to a BigInt value, or throws if an implicit conversion from Number
would be required. It performs the following steps when called:
The abstract operation CanonicalNumericIndexString takes argument argument (a String)
and returns a Number or undefined. If argument is either
"-0" or exactly matches ToString(n) for some Number value
n, it returns the respective Number value. Otherwise, it returns
undefined. It performs the following steps when called:
The abstract operation IsCallable takes argument argument (an ECMAScript language value) and
returns a Boolean. It determines if argument is a callable function with a [[Call]] internal method. It performs the following steps when called:
If argument has a [[Call]] internal method, return
true.
Return false.
7.2.4 IsConstructor ( argument )
The abstract operation IsConstructor takes argument argument (an ECMAScript language value) and
returns a Boolean. It determines if argument is a function object with a [[Construct]] internal method. It performs the following steps when
called:
If argument has a [[Construct]] internal method,
return true.
Return false.
7.2.5 IsExtensible ( O )
The abstract operation IsExtensible takes argument O (an Object) and returns either a
normal completion
containing a Boolean or a throw completion. It is
used to determine whether additional properties can be added to O. It performs the
following steps when called:
The abstract operation IsStringWellFormedUnicode takes argument string (a String) and
returns a Boolean. It interprets string as a sequence of UTF-16 encoded code points,
as described in 6.1.4, and determines
whether it is a well
formed UTF-16 sequence. It performs the following steps when called:
If cp.[[IsUnpairedSurrogate]] is
true, return false.
Set k to k + cp.[[CodeUnitCount]].
Return true.
7.2.8 SameType ( x, y )
The abstract operation SameType takes arguments x (an ECMAScript language value) and
y (an ECMAScript language value) and
returns a Boolean. It determines whether or not the two arguments are the same type. It performs
the following steps when called:
If x is undefined and y is
undefined, return true.
The abstract operation SameValue takes arguments x (an ECMAScript language value) and
y (an ECMAScript language value) and
returns a Boolean. It determines whether or not the two arguments are the same value. It
performs the following steps when called:
This algorithm differs from the IsStrictlyEqual Algorithm by treating
all NaN values as equivalent and by differentiating
+0𝔽 from -0𝔽.
7.2.10 SameValueZero ( x, y )
The abstract operation SameValueZero takes arguments x (an ECMAScript language value) and
y (an ECMAScript language value) and
returns a Boolean. It determines whether or not the two arguments are the same value (ignoring
the difference between +0𝔽 and
-0𝔽). It performs the following steps when called:
SameValueZero differs from SameValue only in that it treats
+0𝔽 and -0𝔽 as equivalent.
7.2.11 SameValueNonNumber ( x, y )
The abstract operation SameValueNonNumber takes arguments x (an ECMAScript language value, but not
a Number) and y (an ECMAScript language value, but not
a Number) and returns a Boolean. It performs the following steps when called:
For expository purposes, some cases are handled separately within this algorithm even if it
is unnecessary to do so.
Note 2
The specifics of what "x is y" means are detailed in 5.2.7.
7.2.12 IsLessThan ( x, y, LeftFirst
)
The abstract operation IsLessThan takes arguments x (an ECMAScript language value),
y (an ECMAScript language value), and
LeftFirst (a Boolean) and returns either a normal completion
containing either a Boolean or undefined, or a
throw completion. It
provides the semantics for the comparison x < y, returning
true, false, or undefined (which
indicates that at least one operand is NaN). The LeftFirst flag is
used to control the order in which operations with potentially visible side-effects are
performed upon x and y. It is necessary because ECMAScript specifies left
to right evaluation of expressions. If LeftFirst is true, the
x parameter corresponds to an expression that occurs to the left of the y
parameter's corresponding expression. If LeftFirst is false, the
reverse is the case and operations must be performed upon y before x. It
performs the following steps when called:
If ℝ(nx) < ℝ(ny), return
true; otherwise return false.
Note 1
Step 3 differs from step 1.c in the algorithm that
handles the addition operator + (13.15.3) by using
the logical-and operation instead of the logical-or operation.
Note 2
The comparison of Strings uses a simple lexicographic ordering on sequences of UTF-16
code unit values. There is no attempt to use the more complex, semantically oriented
definitions of character or string equality and collating order defined in the Unicode
specification. Therefore String values that are canonically equal according to the
Unicode Standard but not in the same normalization form could test as unequal. Also note
that lexicographic ordering by code unit differs from ordering by code
point for Strings containing surrogate pairs.
If x is not finite or y is not finite,
return false.
If ℝ(x)
= ℝ(y),
return true; otherwise return false.
Return false.
7.2.14 IsStrictlyEqual ( x, y )
The abstract operation IsStrictlyEqual takes arguments x (an ECMAScript language value) and
y (an ECMAScript language value) and
returns a Boolean. It provides the semantics for the === operator. It performs the
following steps when called:
This algorithm differs from the SameValue Algorithm in its treatment of
signed zeroes and NaNs.
7.3 Operations on Objects
7.3.1 MakeBasicObject ( internalSlotsList )
The abstract operation MakeBasicObject takes argument internalSlotsList (a List of internal slot
names) and returns an Object. It is the source of all ECMAScript objects that are created
algorithmically, including both ordinary objects and exotic objects.
It factors out common steps used in creating all objects, and centralizes object creation. It
performs the following steps when called:
Set internalSlotsList to the list-concatenation of
internalSlotsList and « [[PrivateElements]] ».
Let obj be a newly created object with an internal slot for each name in
internalSlotsList.
Set obj's essential internal methods to the default ordinary
object definitions specified in 10.1.
Assert: If the
caller will not be overriding both obj's [[GetPrototypeOf]] and [[SetPrototypeOf]] essential internal methods, then
internalSlotsList contains [[Prototype]].
Assert: If the
caller will not be overriding all of obj's [[SetPrototypeOf]], [[IsExtensible]],
and [[PreventExtensions]] essential internal methods, then
internalSlotsList contains [[Extensible]].
If internalSlotsList contains [[Extensible]], set
obj.[[Extensible]] to true.
Return obj.
Note
Within this specification, exotic objects are created in abstract
operations such as ArrayCreate and BoundFunctionCreate by first
calling MakeBasicObject to obtain a basic, foundational object, and then overriding some
or all of that object's internal methods. In order to encapsulate exotic
object creation, the object's essential internal methods are
never modified outside those operations.
The abstract operation Set takes arguments O (an Object), P (a property key),
V (an ECMAScript language value), and
Throw (a Boolean) and returns either a normal completion
containingunused or a throw completion. It is
used to set the value of a specific property of an object. V is the new value for the
property. It performs the following steps when called:
Let success be ? O.[[Set]](P, V, O).
If success is false and Throw is
true, throw a TypeError exception.
Let newDesc be the PropertyDescriptor { [[Value]]:
V, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.
Return ? O.[[DefineOwnProperty]](P,
newDesc).
Note
This abstract operation creates a property whose attributes are set to the same defaults
used for properties created by the ECMAScript language assignment operator. Normally,
the property will not already exist. If it does exist and is not configurable or if
O is not extensible, [[DefineOwnProperty]] will
return false.
7.3.6 CreateDataPropertyOrThrow ( O, P,
V )
The abstract operation CreateDataPropertyOrThrow takes arguments O (an Object),
P (a property
key), and V (an ECMAScript language value) and
returns either a normal completion
containingunused or a throw completion. It is
used to create a new own property of an object. It throws a TypeError
exception if the requested property update cannot be performed. It performs the following steps
when called:
This abstract operation creates a property whose attributes are set to the same defaults
used for properties created by the ECMAScript language assignment operator. Normally,
the property will not already exist. If it does exist and is not configurable or if
O is not extensible, [[DefineOwnProperty]] will
return false causing this operation to throw a
TypeError exception.
7.3.7 CreateNonEnumerableDataPropertyOrThrow ( O,
P, V )
The abstract operation CreateNonEnumerableDataPropertyOrThrow takes arguments O (an
Object), P (a property key), and V (an ECMAScript language value) and
returns unused. It is used to create a new non-enumerable own property of
an ordinary
object. It performs the following steps when called:
Assert:
O is an ordinary, extensible object with no non-configurable properties.
Let newDesc be the PropertyDescriptor { [[Value]]:
V, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.
This abstract operation creates a property whose attributes are set to the same defaults
used for properties created by the ECMAScript language assignment operator except it is
not enumerable. Normally, the property will not already exist. If it does exist,
DefinePropertyOrThrow is
guaranteed to complete normally.
7.3.8 DefinePropertyOrThrow ( O, P,
desc )
The abstract operation DefinePropertyOrThrow takes arguments O (an Object),
P (a property
key), and desc (a Property Descriptor)
and returns either a normal completion
containingunused or a throw completion. It is
used to call the [[DefineOwnProperty]] internal method of an object in
a manner that will throw a TypeError exception if the requested property
update cannot be performed. It performs the following steps when called:
Let success be ? O.[[DefineOwnProperty]](P, desc).
If success is false, throw a TypeError
exception.
Return unused.
7.3.9 DeletePropertyOrThrow ( O, P )
The abstract operation DeletePropertyOrThrow takes arguments O (an Object) and
P (a property
key) and returns either a normal completion
containingunused or a throw completion. It is
used to remove a specific own property of an object. It throws an exception if the property is
not configurable. It performs the following steps when called:
If func is either undefined or null,
return undefined.
If IsCallable(func) is
false, throw a TypeError exception.
Return func.
7.3.11 HasProperty ( O, P )
The abstract operation HasProperty takes arguments O (an Object) and P (a
property
key) and returns either a normal completion
containing a Boolean or a throw completion. It is
used to determine whether an object has a property with the specified property key. The
property may be either own or inherited. It performs the following steps when called:
Return ? O.[[HasProperty]](P).
7.3.12 HasOwnProperty ( O, P )
The abstract operation HasOwnProperty takes arguments O (an Object) and P
(a property
key) and returns either a normal completion
containing a Boolean or a throw completion. It is
used to determine whether an object has an own property with the specified property key. It
performs the following steps when called:
The abstract operation Construct takes argument F (a constructor) and optional arguments
argumentsList (a List of
ECMAScript language values) and
newTarget (a constructor) and returns either a normal completion
containing an Object or a throw completion. It is
used to call the [[Construct]] internal method of a function
object. argumentsList and newTarget are the values
to be passed as the corresponding arguments of the internal method. If argumentsList
is not present, a new empty List is used
as its value. If newTarget is not present, F is used as its value. It
performs the following steps when called:
If newTarget is not present, set newTarget to F.
If argumentsList is not present, set argumentsList to a new empty
List.
If newTarget is not present, this operation is equivalent to:
new F(...argumentsList)
7.3.15 SetIntegrityLevel ( O, level )
The abstract operation SetIntegrityLevel takes arguments O (an Object) and
level (sealed or frozen) and returns
either a normal completion
containing a Boolean or a throw completion. It is
used to fix the set of own properties of an object. It performs the following steps when called:
The abstract operation TestIntegrityLevel takes arguments O (an Object) and
level (sealed or frozen) and returns
either a normal completion
containing a Boolean or a throw completion. It is
used to determine if the set of own properties of an object are fixed. It performs the following
steps when called:
NOTE: If the object is extensible, none of its properties are examined.
Let keys be ? O.[[OwnPropertyKeys]]().
For each element k of keys, do
Let currentDesc be ? O.[[GetOwnProperty]](k).
If currentDesc is not undefined, then
If currentDesc.[[Configurable]] is
true, return false.
If level is frozen and IsDataDescriptor(currentDesc)
is true, then
If currentDesc.[[Writable]]
is true, return false.
Return true.
7.3.17 CreateArrayFromList ( elements )
The abstract operation CreateArrayFromList takes argument elements (a List of ECMAScript language values) and
returns an Array. It is used to create an Array whose elements are provided by
elements. It performs the following steps when called:
The abstract operation LengthOfArrayLike takes argument obj (an Object) and returns
either a normal completion
containing a non-negative integer or a throw completion. It
returns the value of the "length" property of an array-like object. It
performs the following steps when called:
The abstract operation CreateListFromArrayLike takes argument obj (an ECMAScript language value) and
optional argument validElementTypes (all or
property-key) and returns either a normal completion
containing a List of
ECMAScript language values or a
throw completion. It is
used to create a List value whose elements
are provided by the indexed properties of obj. validElementTypes indicates
the types of values that are allowed as elements. It performs the following steps when called:
If validElementTypes is not present, set validElementTypes to
all.
The abstract operation OrdinaryHasInstance takes arguments C (an ECMAScript language value) and
O (an ECMAScript language value) and
returns either a normal completion
containing a Boolean or a throw completion. It
implements the default algorithm for determining if O inherits from the instance
object inheritance path provided by C. It performs the following steps when called:
7.3.22 SpeciesConstructor ( O,
defaultConstructor )
The abstract operation SpeciesConstructor takes arguments O (an Object) and
defaultConstructor (a constructor) and returns either a normal completion
containing a constructor or a throw completion. It is
used to retrieve the constructor that should be used to create new objects
that are derived from O. defaultConstructor is the constructor to use
if a constructor%Symbol.species% property
cannot be found starting from O. It performs the following steps when called:
The target passed in here is always a newly created object which is not directly
accessible in case of an error being thrown.
7.3.26 PrivateElementFind ( O, P )
The abstract operation PrivateElementFind takes arguments O (an Object) and
P (a Private Name) and returns a PrivateElement or
empty. It performs the following steps when called:
If O.[[PrivateElements]] contains a PrivateElementpe such that pe.[[Key]] is P,
then
If entry is not empty, throw a
TypeError exception.
Append PrivateElement {
[[Key]]: P, [[Kind]]:
field, [[Value]]:
value } to O.[[PrivateElements]].
Return unused.
7.3.28 PrivateMethodOrAccessorAdd ( O, method
)
The abstract operation PrivateMethodOrAccessorAdd takes arguments O (an Object) and
method (a PrivateElement) and returns
either a normal completion
containingunused or a throw completion. It
performs the following steps when called:
Assert:
method.[[Kind]] is either
method or accessor.
7.3.33 InitializeInstanceElements ( O,
constructor )
The abstract operation InitializeInstanceElements takes arguments O (an Object) and
constructor (an ECMAScript function object) and returns either a normal completion
containingunused or a throw completion. It
performs the following steps when called:
Let methods be the value of constructor.[[PrivateMethods]].
The abstract operation IteratorComplete takes argument iteratorResult (an Object) and
returns either a normal completion
containing a Boolean or a throw completion. It
performs the following steps when called:
The abstract operation IteratorStep takes argument iteratorRecord (an Iterator
Record) and returns either a normal completion
containing either an Object or done, or a
throw completion. It
requests the next value from iteratorRecord.[[Iterator]] by
calling iteratorRecord.[[NextMethod]] and returns either
done indicating that the iterator has reached its end
or the IteratorResult object if a next
value is available. It performs the following steps when called:
The abstract operation IteratorStepValue takes argument iteratorRecord (an Iterator
Record) and returns either a normal completion
containing either an ECMAScript language
value or done, or a throw completion. It
requests the next value from iteratorRecord.[[Iterator]] by
calling iteratorRecord.[[NextMethod]] and returns either
done indicating that the iterator has reached its end
or the value from the IteratorResult object if a next
value is available. It performs the following steps when called:
The abstract operation IteratorClose takes arguments iteratorRecord (an Iterator
Record) and completion (a Completion Record) and
returns a Completion Record. It is
used to notify an iterator that it should perform any
actions it would normally perform when it has reached its completed state. It performs the
following steps when called:
The abstract operation AsyncIteratorClose takes arguments iteratorRecord (an Iterator
Record) and completion (a Completion Record) and
returns a Completion Record. It is
used to notify an async iterator that it should perform
any actions it would normally perform when it has reached its completed state. It performs the
following steps when called:
If innerResult.[[Value]]is not an
Object, throw a TypeError exception.
Return ? completion.
7.4.14 CreateIteratorResultObject ( value,
done )
The abstract operation CreateIteratorResultObject takes arguments value (an ECMAScript language value) and
done (a Boolean) and returns an Object that conforms to the IteratorResult interface. It creates
an object that conforms to the IteratorResult interface. It
performs the following steps when called:
The abstract operation CreateListIteratorRecord takes argument list (a List of ECMAScript language values) and
returns an Iterator Record. It creates an Iterator
Record whose [[NextMethod]] returns the
successive elements of list. It performs the following steps when called:
Let closure be a new Abstract Closure with no parameters
that captures list and performs the following steps when called:
The definitions for this operation are distributed over the "ECMAScript Language" sections of
this specification. Each definition appears after the defining occurrence of the relevant
productions.
"*default*" is used within this specification as a synthetic name for
a module's default export when it does not have another name. An entry in the module's
[[Environment]] is created with that name and holds the
corresponding value, and resolving the export named "default" by
calling ResolveExport ( exportName [ ,
resolveSet ] ) for the module will return a ResolvedBinding Record whose [[BindingName]] is "*default*", which will
then resolve in the module's [[Environment]] to the
above-mentioned value. This is done only for ease of specification, so that anonymous
default exports can be resolved like any other export. This
"*default*" string is never accessible to ECMAScript code or to the
module linking algorithm.
It is defined piecewise over the following productions:
It is not necessary to treat export defaultAssignmentExpression as a
constant declaration because there is no syntax that permits assignment to the internal
bound name used to reference a module's default object.
8.2.4 Static Semantics: LexicallyDeclaredNames
The syntax-directed
operation LexicallyDeclaredNames takes no arguments and returns a
List of Strings. It is
defined piecewise over the following productions:
The syntax-directed
operation LexicallyScopedDeclarations takes no arguments and returns a
List of Parse
Nodes. It is defined piecewise over the following productions:
The syntax-directed
operation VarDeclaredNames takes no arguments and returns a List of Strings. It is
defined piecewise over the following productions:
The syntax-directed
operation VarScopedDeclarations takes no arguments and returns a
List of Parse
Nodes. It is defined piecewise over the following productions:
The syntax-directed
operation TopLevelLexicallyDeclaredNames takes no arguments and returns a
List of Strings. It is
defined piecewise over the following productions:
The syntax-directed
operation TopLevelLexicallyScopedDeclarations takes no arguments and
returns a List of Parse
Nodes. It is defined piecewise over the following productions:
The syntax-directed
operation TopLevelVarDeclaredNames takes no arguments and returns a
List of Strings. It is
defined piecewise over the following productions:
The syntax-directed
operation TopLevelVarScopedDeclarations takes no arguments and returns a
List of Parse
Nodes. It is defined piecewise over the following productions:
The syntax-directed
operation ContainsDuplicateLabels takes argument labelSet (a
List of Strings) and
returns a Boolean. It is defined piecewise over the following productions:
The syntax-directed
operation ContainsUndefinedBreakTarget takes argument labelSet
(a List of Strings) and
returns a Boolean. It is defined piecewise over the following productions:
The syntax-directed
operation ContainsUndefinedContinueTarget takes arguments
iterationSet (a List of
Strings) and labelSet (a List of
Strings) and returns a Boolean. It is defined piecewise over the following productions:
The abstract operation IsAnonymousFunctionDefinition takes argument expr (an AssignmentExpressionParse
Node, an InitializerParse
Node, or an ExpressionParse Node) and returns a
Boolean. It determines if its argument is a function definition that does not bind a name. It
performs the following steps when called:
The syntax-directed
operation ComputedPropertyContains takes argument symbol (a
grammar symbol) and returns a Boolean. It is defined piecewise over the following productions:
undefined is passed for environment to indicate that a
PutValue
operation should be used to assign the initialization value. This is the case for
var statements and formal parameter lists of some non-strict
functions (See 10.2.11). In those
cases a lexical binding is hoisted and preinitialized prior to evaluation of its
initializer.
It is defined piecewise over the following productions:
When undefined is passed for environment it indicates that
a PutValue operation should be used to assign
the initialization value. This is the case for formal parameter lists of non-strict
functions. In that case the formal parameter bindings are
preinitialized in order to deal with the possibility of multiple parameters with the
same name.
It is defined piecewise over the following productions:
The syntax-directed
operation AssignmentTargetType takes no arguments and returns
simple or invalid. It is defined piecewise over
the following productions:
Environment Record is a specification type
used to define the association of Identifiers to specific variables and functions, based
upon the lexical nesting structure of ECMAScript code. Usually an Environment Record is associated
with some specific syntactic structure of ECMAScript code such as a FunctionDeclaration, a BlockStatement, or a Catch clause of a TryStatement. Each time such code is evaluated, a new
Environment Record is created to record the identifier bindings that are created by that code.
Every Environment Record has an [[OuterEnv]] field, which is either
null or a reference to an outer Environment Record. This is used to model the
logical nesting of Environment Record values. The outer reference of an (inner) Environment Record
is a reference to the Environment Record that logically surrounds the inner Environment Record. An
outer Environment Record may, of course, have its own outer Environment Record. An Environment
Record may serve as the outer environment for multiple inner Environment Records. For example, if a
FunctionDeclaration
contains two nested FunctionDeclarations then the Environment
Records of each of the nested functions will have as their outer Environment Record the Environment
Record of the current evaluation of the surrounding function.
Environment Records are purely specification mechanisms and need not correspond to any specific
artefact of an ECMAScript implementation. It is impossible for an ECMAScript program to directly
access or manipulate such values.
A Function
Environment Record corresponds to the
invocation of an ECMAScript function
object, and contains bindings for the top-level
declarations within that function. It may establish a new
this binding. It also captures the state necessary to
support super method invocations.
An Object Environment
Record is used to define the effect of
ECMAScript elements such as WithStatement that associate
identifier bindings with the properties of some object.
A Global Environment
Record is used for Script global declarations. It does not
have an outer environment; its [[OuterEnv]] is
null. It may be prepopulated with identifier bindings and it
includes an associated global object whose properties
provide some of the global environment's identifier bindings. As ECMAScript code
is executed, additional properties may be added to the global
object and the initial properties may be modified.
The Environment Record abstract class
includes the abstract specification methods defined in Table 16. These
abstract methods have distinct concrete algorithms for each of the concrete subclasses.
Determine if an Environment Record
has a binding for the String value N. Return
true if it does and false if it does
not.
CreateMutableBinding(N, D)
Create a new but uninitialized mutable binding in an Environment Record.
The String value N is the text of the bound name. If the Boolean
argument D is true the binding may be
subsequently deleted.
CreateImmutableBinding(N, S)
Create a new but uninitialized immutable binding in an Environment Record.
The String value N is the text of the bound name. If S
is true then attempts to set it after it has been
initialized will always throw an exception, regardless of the strict mode
setting of operations that reference that binding.
InitializeBinding(N, V)
Set the value of an already existing but uninitialized binding in an
Environment Record.
The String value N is the text of the bound name. V is
the value for the binding and is a value of any ECMAScript language
type.
SetMutableBinding(N, V, S)
Set the value of an already existing mutable binding in an Environment Record.
The String value N is the text of the bound name. V is
the value for the binding and may be a value of any ECMAScript language
type. Sis a
Boolean flag. If S is
true and the binding cannot be set throw a
TypeError exception.
GetBindingValue(N, S)
Returns the value of an already existing binding from an Environment Record.
The String value N is the text of the bound name. S is
used to identify references originating in strict mode code or that
otherwise require strict mode reference semantics. If S is
true and the binding does not exist throw a
ReferenceError exception. If the binding exists but is
uninitialized a ReferenceError is thrown, regardless of
the value of S.
DeleteBinding(N)
Delete a binding from an Environment
Record. The String value N is the text of
the bound name. If a binding for N exists, remove the binding and
return true. If the binding exists but cannot be removed
return false. If the binding does not exist return
true.
HasThisBinding()
Determine if an Environment Record
establishes a this binding. Return true if
it does and false if it does not.
HasSuperBinding()
Determine if an Environment Record
establishes a super method binding. Return
true if it does and false if it does
not. If it returns true it implies that the Environment Record is
a Function Environment
Record, although the reverse implication does not
hold.
WithBaseObject()
If this Environment Record is
associated with a with statement, return the with object.
Otherwise, return undefined.
9.1.1.1 Declarative Environment Records
Each Declarative Environment
Record is associated with an ECMAScript program scope containing variable,
constant, let, class, module, import, and/or function declarations. A Declarative
Environment Record binds the set of identifiers defined by the declarations contained within
its scope.
9.1.1.1.1 HasBinding ( N )
The HasBinding concrete method of a Declarative
Environment RecordenvRec takes argument N
(a String) and returns a normal completion
containing a Boolean. It determines if the argument identifier is
one of the identifiers bound by the record. It performs the following steps when called:
If envRec has a binding for N, return
true.
Return false.
9.1.1.1.2 CreateMutableBinding ( N, D
)
The CreateMutableBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String)
and D (a Boolean) and returns a normal completion
containingunused. It creates a new
mutable binding for the name N that is uninitialized. A binding must not
already exist in this Environment Record for
N. If D is true, the new binding is marked as
being subject to deletion. It performs the following steps when called:
Assert:
envRec does not already have a binding for N.
Create a mutable binding in envRec for N and record that
it is uninitialized. If D is true, record that the
newly created binding may be deleted by a subsequent DeleteBinding call.
Return unused.
9.1.1.1.3 CreateImmutableBinding ( N,
S )
The CreateImmutableBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String)
and S (a Boolean) and returns a normal completion
containingunused. It creates a new
immutable binding for the name N that is uninitialized. A binding must not
already exist in this Environment Record for
N. If S is true, the new binding is marked as a
strict binding. It performs the following steps when called:
Assert:
envRec does not already have a binding for N.
Create an immutable binding in envRec for N and record
that it is uninitialized. If S is true, record
that the newly created binding is a strict binding.
Return unused.
9.1.1.1.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String)
and V (an ECMAScript language value)
and returns a normal completion
containingunused. It is used to set the
bound value of the current binding of the identifier whose name is N to the
value V. An uninitialized binding for N must already exist. It
performs the following steps when called:
Assert:
envRec must have an uninitialized binding for N.
Set the bound value for N in envRec to V.
Record that the binding for N in
envRec has been initialized.
Return unused.
9.1.1.1.5 SetMutableBinding ( N, V,
S )
The SetMutableBinding concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String),
V (an ECMAScript language value),
and S (a Boolean) and returns either a normal completion
containingunused or a throw
completion. It attempts to change the bound value of the current
binding of the identifier whose name is N to the value V. A
binding for N normally already exists, but in rare cases it may not. If the
binding is an immutable binding, a TypeError is thrown if
S is true. It performs the following steps when called:
If envRec does not have a
binding for N, then
If S is true, throw a
ReferenceError exception.
Perform ! envRec.CreateMutableBinding(N,
true).
Perform ! envRec.InitializeBinding(N,
V).
Return unused.
If the binding for N in envRec is a strict binding, set
S to true.
If the binding for N in envRec has not yet been
initialized, then
Throw a ReferenceError exception.
Else if the binding for N in envRec is a mutable binding,
then
Change its bound value to V.
Else,
Assert: This is an attempt to
change the value of an immutable binding.
If S is true, throw a
TypeError exception.
Return unused.
Note
An example of ECMAScript code that results in a missing binding at step 1 is:
functionf() { eval("var x; x = (delete x, 0);"); }
9.1.1.1.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Declarative Environment
RecordenvRec takes arguments N (a String)
and S (a Boolean) and returns either a normal completion
containing an ECMAScript language
value or a throw
completion. It returns the value of its bound identifier whose
name is N. If the binding exists but is uninitialized a
ReferenceError is thrown, regardless of the value of S. It
performs the following steps when called:
If the binding for N in envRec is an uninitialized
binding, throw a ReferenceError exception.
Return the value currently bound to N in envRec.
9.1.1.1.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Declarative Environment
RecordenvRec takes argument N (a String)
and returns a normal completion
containing a Boolean. It can only delete bindings that have been
explicitly designated as being subject to deletion. It performs the following steps when
called:
If the binding for N in envRec cannot be deleted, return
false.
Remove the binding for N from envRec.
Return true.
9.1.1.1.8 HasThisBinding ( )
The HasThisBinding concrete method of a Declarative Environment
RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The HasSuperBinding concrete method of a Declarative Environment
RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The WithBaseObject concrete method of a Declarative Environment
RecordenvRec takes no arguments and returns
undefined. It performs the following steps when called:
Return undefined.
9.1.1.2 Object Environment Records
Each Object Environment Record
is associated with an object called its binding object. An Object Environment
Record binds the set of string identifier names that directly correspond to the property
names of its binding object. Property keys that are not strings in the form
of an IdentifierName are
not included in the set of bound identifiers. Both own and inherited properties are included
in the set regardless of the setting of their [[Enumerable]]
attribute. Because properties can be dynamically added and deleted from objects, the set of
identifiers bound by an Object Environment Record may potentially change as a side-effect of
any operation that adds or deletes properties. Any bindings that are created as a result of
such a side-effect are considered to be a mutable binding even if the Writable attribute of
the corresponding property is false. Immutable bindings do not exist for
Object Environment Records.
Object Environment Records created for with statements (14.11) can provide their binding object as
an implicit this value for use in function calls. The capability is
controlled by a Boolean [[IsWithEnvironment]] field.
Object Environment Records have the additional state fields listed in Table
17.
Indicates whether this Environment
Record is created for a with
statement.
9.1.1.2.1 HasBinding ( N )
The HasBinding concrete method of an Object Environment
RecordenvRec takes argument N (a String)
and returns either a normal completion
containing a Boolean or a throw
completion. It determines if its associated binding object has a
property whose name is N. It performs the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Let foundBinding be ? HasProperty(bindingObject,
N).
If foundBinding is false, return
false.
If envRec.[[IsWithEnvironment]] is
false, return true.
The CreateMutableBinding concrete method of an Object Environment RecordenvRec takes arguments N (a String) and D (a Boolean)
and returns either a normal completion
containingunused or a throw
completion. It creates in an Environment Record's associated
binding object a property whose name is N and initializes it to the value
undefined. If D is true, the new
property's [[Configurable]] attribute is set to
true; otherwise it is set to false. It performs
the following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Perform ? DefinePropertyOrThrow(bindingObject,
N, PropertyDescriptor { [[Value]]:
undefined, [[Writable]]:
true, [[Enumerable]]:
true, [[Configurable]]:
D }).
Return unused.
Note
Normally envRec will not have a binding for N but if it
does, the semantics of DefinePropertyOrThrow
may result in an existing binding being replaced or shadowed or cause an
abrupt
completion to be returned.
9.1.1.2.3 CreateImmutableBinding ( N,
S )
The CreateImmutableBinding concrete method of an Object Environment Record
is never used within this specification.
In this specification, all uses of CreateMutableBinding for Object Environment
Records are immediately followed by a call to
InitializeBinding for the same name. Hence, this specification does not
explicitly track the initialization state of bindings in Object Environment
Records.
9.1.1.2.5 SetMutableBinding ( N, V,
S )
The SetMutableBinding concrete method of an Object Environment RecordenvRec takes arguments N (a String), V (an ECMAScript language value),
and S (a Boolean) and returns either a normal completion
containingunused or a throw
completion. It attempts to set the value of the Environment Record's associated
binding object's property whose name is N to the value V. A
property named N normally already exists but if it does not or is not
currently writable, error handling is determined by S. It performs the
following steps when called:
Let bindingObject be envRec.[[BindingObject]].
Let stillExists be ? HasProperty(bindingObject,
N).
If stillExists is false and S is
true, throw a ReferenceError exception.
The GetBindingValue concrete method of an Object Environment RecordenvRec takes arguments N (a String) and S (a Boolean)
and returns either a normal completion
containing an ECMAScript language
value or a throw
completion. It returns the value of its associated binding
object's property whose name is N. The property should already exist but if
it does not the result depends upon S. It performs the following steps when
called:
The DeleteBinding concrete method of an Object Environment
RecordenvRec takes argument N (a String)
and returns either a normal completion
containing a Boolean or a throw
completion. It can only delete bindings that correspond to
properties of the environment object whose [[Configurable]]
attribute have the value true. It performs the following steps when
called:
Let bindingObject be envRec.[[BindingObject]].
Return ? bindingObject.[[Delete]](N).
9.1.1.2.8 HasThisBinding ( )
The HasThisBinding concrete method of an Object Environment
RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The HasSuperBinding concrete method of an Object Environment RecordenvRec takes no arguments and returns false. It performs
the following steps when called:
The WithBaseObject concrete method of an Object Environment
RecordenvRec takes no arguments and returns an Object
or undefined. It performs the following steps when called:
If envRec.[[IsWithEnvironment]] is
true, return envRec.[[BindingObject]].
Otherwise, return undefined.
9.1.1.3 Function Environment Records
A Function Environment
Record is a Declarative Environment
Record that is used to represent the top-level scope of a function
and, if the function is not an ArrowFunction, provides a this
binding. If a function is not an ArrowFunction function and references
super, its Function Environment Record also contains the state that is used to
perform super method invocations from within the function.
Function Environment Records have the additional state fields listed in Table
18.
If this Environment
Record was created by the [[Construct]] internal method, [[NewTarget]] is the value of the [[Construct]]newTarget parameter.
Otherwise, its value is undefined.
Function Environment Records support all of the Declarative Environment
Record methods listed in Table 16 and
share the same specifications for all of those methods except for HasThisBinding and
HasSuperBinding. In addition, Function Environment Records support the methods listed in
Table
19:
Assert:
envRec.[[ThisBindingStatus]] is not
lexical.
If envRec.[[ThisBindingStatus]] is
initialized, throw a ReferenceError
exception.
Set envRec.[[ThisValue]] to V.
Set envRec.[[ThisBindingStatus]] to
initialized.
Return unused.
9.1.1.3.2 HasThisBinding ( )
The HasThisBinding concrete method of a Function Environment
RecordenvRec takes no arguments and returns a
Boolean. It performs the following steps when called:
If envRec.[[ThisBindingStatus]] is
lexical, return false; otherwise,
return true.
9.1.1.3.3 HasSuperBinding ( )
The HasSuperBinding concrete method of a Function Environment
RecordenvRec takes no arguments and returns a
Boolean. It performs the following steps when called:
If envRec.[[ThisBindingStatus]] is
lexical, return false.
If envRec.[[FunctionObject]].[[HomeObject]] is undefined, return
false; otherwise, return true.
Assert:
envRec.[[ThisBindingStatus]] is not
lexical.
If envRec.[[ThisBindingStatus]] is
uninitialized, throw a ReferenceError
exception.
Return envRec.[[ThisValue]].
9.1.1.3.5 GetSuperBase ( envRec )
The abstract operation GetSuperBase takes argument envRec (a Function Environment
Record) and returns an Object, null, or
undefined. It returns the object that is the base for
super property accesses bound in envRec. The value
undefined indicates that such accesses will produce runtime errors.
It performs the following steps when called:
Let home be envRec.[[FunctionObject]].[[HomeObject]].
A Global Environment Record is
used to represent the outer most scope that is shared by all of the ECMAScript Script elements that are processed
in a common realm. A
Global Environment Record provides the bindings for built-in globals (clause 19),
properties of the global object, and for all top-level
declarations (8.2.9,
8.2.11)
that occur within a Script.
The HasBinding concrete method of a Global Environment
RecordenvRec takes argument N (a String)
and returns either a normal completion
containing a Boolean or a throw
completion. It determines if the argument identifier is one of
the identifiers bound by the record. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true,
return true.
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.HasBinding(N).
9.1.1.4.2 CreateMutableBinding ( N, D
)
The CreateMutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and D (a Boolean)
and returns either a normal completion
containingunused or a throw
completion. It creates a new mutable binding for the name
N that is uninitialized. The binding is created in the associated
DeclarativeRecord. A binding for N must not already exist in the
DeclarativeRecord. If D is true, the new binding is marked
as being subject to deletion. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true,
throw a TypeError exception.
Return ! DclRec.CreateMutableBinding(N,
D).
9.1.1.4.3 CreateImmutableBinding ( N,
S )
The CreateImmutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and S (a Boolean)
and returns either a normal completion
containingunused or a throw
completion. It creates a new immutable binding for the name
N that is uninitialized. A binding must not already exist in this Environment Record for
N. If S is true, the new binding is marked as a
strict binding. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true,
throw a TypeError exception.
Return ! DclRec.CreateImmutableBinding(N,
S).
9.1.1.4.4 InitializeBinding ( N, V )
The InitializeBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String) and V (an ECMAScript language value)
and returns either a normal completion
containingunused or a throw
completion. It is used to set the bound value of the current
binding of the identifier whose name is N to the value V. An
uninitialized binding for N must already exist. It performs the following
steps when called:
The SetMutableBinding concrete method of a Global Environment RecordenvRec takes arguments N (a String), V (an ECMAScript language value),
and S (a Boolean) and returns either a normal completion
containingunused or a throw
completion. It attempts to change the bound value of the current
binding of the identifier whose name is N to the value V. If the
binding is an immutable binding and S is true, a
TypeError is thrown. A property named N normally already
exists but if it does not or is not currently writable, error handling is determined by
S. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true,
then
Return ? DclRec.SetMutableBinding(N,
V, S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.SetMutableBinding(N,
V, S).
9.1.1.4.6 GetBindingValue ( N, S )
The GetBindingValue concrete method of a Global Environment
RecordenvRec takes arguments N (a String)
and S (a Boolean) and returns either a normal completion
containing an ECMAScript language
value or a throw
completion. It returns the value of its bound identifier whose
name is N. If the binding is an uninitialized binding throw a
ReferenceError exception. A property named N normally
already exists but if it does not or is not currently writable, error handling is
determined by S. It performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
If ! DclRec.HasBinding(N) is true,
then
Return ? DclRec.GetBindingValue(N,
S).
Let ObjRec be envRec.[[ObjectRecord]].
Return ? ObjRec.GetBindingValue(N,
S).
9.1.1.4.7 DeleteBinding ( N )
The DeleteBinding concrete method of a Global Environment
RecordenvRec takes argument N (a String)
and returns either a normal completion
containing a Boolean or a throw
completion. It can only delete bindings that have been explicitly
designated as being subject to deletion. It performs the following steps when called:
The HasThisBinding concrete method of a Global Environment
RecordenvRec takes no arguments and returns
true. It performs the following steps when called:
The HasSuperBinding concrete method of a Global Environment
RecordenvRec takes no arguments and returns
false. It performs the following steps when called:
The WithBaseObject concrete method of a Global Environment
RecordenvRec takes no arguments and returns
undefined. It performs the following steps when called:
The abstract operation HasLexicalDeclaration takes arguments envRec (a
Global Environment Record)
and N (a String) and returns a Boolean. It determines if the argument
identifier has a binding in envRec that was created using a lexical
declaration such as a LexicalDeclaration or a ClassDeclaration. It
performs the following steps when called:
Let DclRec be envRec.[[DeclarativeRecord]].
Return ! DclRec.HasBinding(N).
9.1.1.4.13 HasRestrictedGlobalProperty ( envRec,
N )
The abstract operation HasRestrictedGlobalProperty takes arguments envRec (a
Global Environment Record)
and N (a String) and returns either a normal completion
containing a Boolean or a throw
completion. It determines if the argument identifier is the name
of a property of the global object that must not be shadowed
by a global lexical binding. It performs the following steps when called:
Let ObjRec be envRec.[[ObjectRecord]].
Let globalObject be ObjRec.[[BindingObject]].
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return
false.
If existingProp.[[Configurable]] is
true, return false.
Return true.
Note
Properties may exist upon a global object that were
directly created rather than being declared using a var or function declaration.
A global lexical binding may not be created that has the same name as a
non-configurable property of the global object.
The global property "undefined" is an example of such a
property.
9.1.1.4.14 CanDeclareGlobalVar ( envRec,
N )
The abstract operation CanDeclareGlobalVar takes arguments envRec (a Global Environment Record)
and N (a String) and returns either a normal completion
containing a Boolean or a throw
completion. It determines if a corresponding CreateGlobalVarBinding call
would succeed if called for the same argument N. Redundant var declarations
and var declarations for pre-existing global object properties
are allowed. It performs the following steps when called:
Let existingProp be ? globalObject.[[GetOwnProperty]](N).
If existingProp is undefined, return
? IsExtensible(globalObject).
If existingProp.[[Configurable]] is
true, return true.
If IsDataDescriptor(existingProp)
is true and existingProp has attribute values {
[[Writable]]: true, [[Enumerable]]: true }, return
true.
Return false.
9.1.1.4.16 CreateGlobalVarBinding ( envRec,
N, D )
The abstract operation CreateGlobalVarBinding takes arguments envRec (a
Global Environment
Record), N (a String), and D (a Boolean)
and returns either a normal completion
containingunused or a throw
completion. It creates and initializes a mutable binding in the
associated Object Environment Record.
If a binding already exists, it is reused and assumed to be initialized. It performs the
following steps when called:
Global function declarations are always represented as own properties of the
global object. If possible, an
existing own property is reconfigured to have a standard set of attribute
values. Step 7 is
equivalent to what calling the InitializeBinding concrete method would do and if
globalObject is a Proxy will produce the same sequence of Proxy trap
calls.
9.1.1.5 Module Environment Records
A Module Environment Record is
a Declarative Environment
Record that is used to represent the outer scope of an ECMAScript
Module. In additional to normal
mutable and immutable bindings, Module Environment Records also provide immutable import
bindings which are bindings that provide indirect access to a target binding that exists in
another Environment Record.
Module Environment Records support all of the Declarative Environment
Record methods listed in Table 16 and
share the same specifications for all of those methods except for GetBindingValue,
DeleteBinding, HasThisBinding and GetThisBinding. In addition, Module Environment Records
support the methods listed in Table
22:
The GetBindingValue concrete method of a Module Environment
RecordenvRec takes arguments N (a String)
and S (a Boolean) and returns either a normal completion
containing an ECMAScript language
value or a throw
completion. It returns the value of its bound identifier whose
name is N. However, if the binding is an indirect binding the value of the
target binding is returned. If the binding exists but is uninitialized a
ReferenceError is thrown. It performs the following steps when
called:
The HasThisBinding concrete method of a Module Environment
RecordenvRec takes no arguments and returns
true. It performs the following steps when called:
9.1.1.5.5 CreateImportBinding ( envRec,
N, M, N2 )
The abstract operation CreateImportBinding takes arguments envRec (a Module Environment
Record), N (a String), M (a Module Record), and
N2 (a String) and returns unused. It creates a new
initialized immutable indirect binding for the name N. A binding must not
already exist in envRec for N. N2 is the name of a
binding that exists in M's Module Environment
Record. Accesses to the value of the new binding will indirectly
access the bound value of the target binding. It performs the following steps when
called:
Assert:
envRec does not already have a binding for N.
Assert:
When M.[[Environment]] is instantiated, it
will have a direct binding for N2.
Create an immutable indirect binding in envRec for N that
references M and N2 as its target binding and record that
the binding is initialized.
The abstract operation NewObjectEnvironment takes arguments O (an Object),
W (a Boolean), and E (an Environment Record or
null) and returns an Object Environment
Record. It performs the following steps when called:
The abstract operation NewFunctionEnvironment takes arguments F (an ECMAScript
function
object) and newTarget (an Object or
undefined) and returns a Function Environment Record.
It performs the following steps when called:
If F.[[ThisMode]] is
lexical, set env.[[ThisBindingStatus]] to lexical.
Else, set env.[[ThisBindingStatus]] to
uninitialized.
Set env.[[NewTarget]] to newTarget.
Set env.[[OuterEnv]] to F.[[Environment]].
Return env.
9.1.2.5 NewGlobalEnvironment ( G,
thisValue )
The abstract operation NewGlobalEnvironment takes arguments G (an Object) and
thisValue (an Object) and returns a Global Environment Record. It
performs the following steps when called:
The abstract operation ResolvePrivateIdentifier takes arguments privateEnv (a
PrivateEnvironment Record) and
identifier (a String) and returns a Private Name. It performs the
following steps when called:
Before it is evaluated, all ECMAScript code must be associated with a realm. Conceptually, a realm consists of a set of intrinsic objects, an
ECMAScript global environment, all of the ECMAScript code that is loaded within the scope of that
global environment, and other associated state and resources.
A realm is represented in this
specification as a Realm Record
with the fields specified in Table 24:
Template objects are canonicalized separately for each realm using
its Realm Record's [[TemplateMap]]. Each [[Site]] value is a Parse Node that is a
TemplateLiteral. The
associated [[Array]] value is the corresponding
template object that is passed to a tag function.
Note 1
Once a Parse
Node becomes unreachable, the corresponding [[Array]] is also unreachable, and it would be
unobservable if an implementation removed the pair from the [[TemplateMap]] list.
A map from the specifier strings imported by this realm to the resolved
Module Record.
The list does not contain two different Recordsr1 and r2 such that ModuleRequestsEqual(r1,
r2) is true.
Field reserved for use by hosts that need to associate additional
information with a Realm Record.
9.3.1 InitializeHostDefinedRealm ( )
The abstract operation InitializeHostDefinedRealm takes no arguments and returns either a
normal completion
containingunused or a throw completion. It
performs the following steps when called:
Set fields of realmRec.[[Intrinsics]] with the values listed in Table 6. The field
names are the names listed in column one of the table. The value of each field is a new
object value fully and recursively populated with property values as defined by the
specification of each object in clauses 19 through 28.
All object property values are newly created object values. All values that are built-in
function
objects are created by performing CreateBuiltinFunction(steps,
length, name, slots, realmRec,
prototype) where steps is the definition of that function provided
by this specification, name is the initial value of the function's
"name" property, length is the initial value of the
function's "length" property, slots is a list of the
names, if any, of the function's specified internal slots, and prototype is
the specified value of the function's [[Prototype]] internal
slot. The creation of the intrinsics and their properties must be ordered to avoid any
dependencies upon objects that have not yet been created.
Let desc be the fully populated data Property
Descriptor for the property, containing the specified
attributes for the property. For properties listed in 19.2,
19.3,
or 19.4
the value of the [[Value]] attribute is the
corresponding intrinsic object from realmRec.
An execution context is a specification device
that is used to track the runtime evaluation of code by an ECMAScript implementation. At any point
in time, there is at most one execution context per agent that is actually executing code. This is known as the
agent's running
execution context. All references to the running execution context in
this specification denote the running execution context of the surrounding
agent.
The execution
context stack is used to track execution contexts. The running
execution context is always the top element of this stack. A new execution
context is created whenever control is transferred from the executable code associated with the
currently running execution context to executable
code that is not associated with that execution context. The newly created execution context is
pushed onto the stack and becomes the running execution context.
An execution context contains whatever implementation specific state is necessary to track the
execution progress of its associated code. Each execution context has at least the state components
listed in Table 25.
Table 25: State Components for All Execution Contexts
Component
Purpose
code evaluation state
Any state needed to perform, suspend, and resume evaluation of the code
associated with this execution context.
Evaluation of
code by the running execution context may be suspended
at various points defined within this specification. Once the running
execution context has been suspended a different execution context may become
the running execution context and commence
evaluating its code. At some later time a suspended execution context may again become the running
execution context and continue evaluating its code at the point where it had
previously been suspended. Transition of the running execution context
status among execution contexts usually occurs in stack-like last-in/first-out manner. However, some
ECMAScript features require non-LIFO transitions of the running execution context.
In most situations only the running execution context (the top of the
execution context stack) is directly
manipulated by algorithms within this specification. Hence when the terms “LexicalEnvironment”, and
“VariableEnvironment” are used without qualification they are in reference to those components of
the running execution context.
An execution context is purely a specification mechanism and need not correspond to any particular
artefact of an ECMAScript implementation. It is impossible for ECMAScript code to directly access or
observe an execution context.
9.4.1 GetActiveScriptOrModule ( )
The abstract operation GetActiveScriptOrModule takes no arguments and returns a Script Record,
a Module Record, or
null. It is used to determine the running script or module, based on the
running execution context. It performs
the following steps when called:
If no such execution context exists, return
null. Otherwise, return ec's ScriptOrModule.
9.4.2 ResolveBinding ( name [ , env ] )
The abstract operation ResolveBinding takes argument name (a String) and optional
argument env (an Environment Record or
undefined) and returns either a normal completion
containing a Reference
Record or a throw
completion. It is used to determine the binding of name.
env can be used to explicitly provide the Environment Record that is
to be searched for the binding. It performs the following steps when called:
The result of ResolveBinding is always a Reference Record
whose [[ReferencedName]] field is name.
9.4.3 GetThisEnvironment ( )
The abstract operation GetThisEnvironment takes no arguments and returns an Environment Record. It finds the
Environment Record that currently
supplies the binding of the keywordthis. It
performs the following steps when called:
The abstract operation GetNewTarget takes no arguments and returns an Object or
undefined. It determines the NewTarget value using the LexicalEnvironment of
the running execution context. It performs
the following steps when called:
The abstract operation GetGlobalObject takes no arguments and returns an Object. It returns the
global
object used by the currently running execution
context. It performs the following steps when called:
A Job is an Abstract Closure with no
parameters that initiates an ECMAScript computation when no other ECMAScript computation is
currently in progress.
At some future point in time, when there is no running context in the agent for which the job is scheduled and that
agent's execution
context stack is empty, the implementation must:
Host
environments are not required to treat Jobs uniformly with respect to scheduling. For
example, web browsers and Node.js treat Promise-handling Jobs as a higher priority than other work; future features
may add Jobs that are not
treated at such a high priority.
At any particular time, scriptOrModule (a Script Record, a Module
Record, or null) is the active script or module if all of the following conditions are true:
The specific choice of Realm is up to the host environment. This initial
execution context and Realm is only in use before
any callback function is invoked. When a callback function related to a Job, like a Promise handler, is
invoked, the invocation pushes its own execution context and
Realm.
Particular kinds of Jobs have
additional conformance requirements.
The WHATWG HTML specification (https://html.spec.whatwg.org/), for
example, uses the host-defined value to propagate the
incumbent settings object for Promise callbacks.
JobCallback Records have the fields listed in Table 28.
An implementation of HostMakeJobCallback must conform to the following requirements:
It must return a JobCallback Record whose [[Callback]] field is callback.
The default implementation of HostMakeJobCallback performs the following steps when called:
Return the JobCallback Record { [[Callback]]: callback, [[HostDefined]]: empty }.
ECMAScript hosts that are
not web browsers must use the default implementation of HostMakeJobCallback.
Note
This is called at the time that the callback is passed to the function that is
responsible for its being eventually scheduled and run. For example,
promise.then(thenAction) calls MakeJobCallback on thenAction
at the time of invoking Promise.prototype.then, not at the time of
scheduling the reaction Job.
ECMAScript hosts that are
not web browsers must use the default implementation of HostCallJobCallback.
9.5.4 HostEnqueueGenericJob ( job, realm )
The host-defined abstract operation
HostEnqueueGenericJob takes arguments job (a JobAbstract Closure) and realm (a
Realm
Record) and returns unused. It schedules
job in the realmrealm in the agent signified by realm.[[AgentSignifier]] to be performed at some future time. The Abstract
Closures used with this algorithm are intended to be scheduled without
additional constraints, such as priority and ordering.
An implementation of HostEnqueueGenericJob must conform to the requirements in 9.5.
9.5.5 HostEnqueuePromiseJob ( job, realm )
The host-defined abstract operation
HostEnqueuePromiseJob takes arguments job (a JobAbstract Closure) and realm (a
Realm
Record or null) and returns
unused. It schedules job to be performed at some future time.
The Abstract Closures used with this algorithm
are intended to be related to the handling of Promises, or otherwise, to be scheduled with equal
priority to Promise handling operations.
An implementation of HostEnqueuePromiseJob must conform to the requirements in 9.5 as well as the
following:
Let scriptOrModule be GetActiveScriptOrModule() at the
time HostEnqueuePromiseJob is invoked. If realm is not null,
each time job is invoked the implementation must perform implementation-defined steps such that
scriptOrModule is the active script or
module at the time of job's invocation.
Jobs must run in the same
order as the HostEnqueuePromiseJob invocations that scheduled them.
Note
The realm for Jobs returned by NewPromiseResolveThenableJob
is usually the result of calling GetFunctionRealm on the
thenfunction object. The realm for
Jobs returned by
NewPromiseReactionJob is
usually the result of calling GetFunctionRealm on the handler if
the handler is not undefined. If the handler is
undefined, realm is null. For both
kinds of Jobs, when
GetFunctionRealm completes
abnormally (i.e. called on a revoked Proxy), realm is the current Realm
Record at the time of the GetFunctionRealm call.
When the realm is null, no user ECMAScript code will be
evaluated and no new ECMAScript objects (e.g. Error objects) will be created. The WHATWG
HTML specification (https://html.spec.whatwg.org/), for
example, uses realm to check for the ability to run script and for the entry concept.
The host-defined abstract operation
HostEnqueueTimeoutJob takes arguments timeoutJob (a JobAbstract Closure),
realm (a Realm
Record), and milliseconds (a non-negative finite Number) and returns
unused. It schedules timeoutJob in the realmrealm in the agent signified by
realm.[[AgentSignifier]] to be performed after at least
milliseconds milliseconds.
An implementation of HostEnqueueTimeoutJob must conform to the requirements in 9.5.
The default value computed for the isLittleEndian parameter when it is
needed by the algorithms GetValueFromBuffer and
SetValueInBuffer. The choice
is implementation-defined and
should be the alternative that is most efficient for the implementation. Once
the value has been observed it cannot change.
Initially 0, used to assign unique incrementing values to the [[AsyncEvaluationOrder]] field of modules that are
asynchronous or have asynchronous dependencies.
Once the values of [[Signifier]], [[IsLockFree1]],
and [[IsLockFree2]] have been observed by any agent in the agent cluster they cannot change.
Note 2
The values of [[IsLockFree1]] and [[IsLockFree2]] are not necessarily determined by the hardware, but
may also reflect implementation choices that can vary over time and between ECMAScript
implementations.
There is no [[IsLockFree4]] field: 4-byte atomic operations are
always lock-free.
In practice, if an atomic operation is implemented with any type of lock the operation is not
lock-free. Lock-free does not imply wait-free: there is no upper bound on how many machine
steps may be required to complete a lock-free atomic operation.
That an atomic access of size n is lock-free does not imply anything about the
(perceived) atomicity of non-atomic accesses of size n, specifically, non-atomic
accesses may still be performed as a sequence of several separate memory accesses. See
ReadSharedMemory and WriteSharedMemory for details.
Note 3
An agent is a
specification mechanism and need not correspond to any particular artefact of an ECMAScript
implementation.
9.6.1 AgentSignifier ( )
The abstract operation AgentSignifier takes no arguments and returns an agent signifier. It
performs the following steps when called:
In some environments it may not be reasonable for a given agent to suspend. For example, in a
web browser environment, it may be reasonable to disallow suspending a document's main
event handling thread, while still allowing workers' event handling threads to suspend.
9.6.3 IncrementModuleAsyncEvaluationCount ( )
The abstract operation IncrementModuleAsyncEvaluationCount takes no arguments and returns an
integer. It performs
the following steps when called:
Set AR.[[ModuleAsyncEvaluationCount]] to
count + 1.
Return count.
Note
This value is only used to keep track of the relative evaluation order between pending
modules. An implementation may unobservably reset [[ModuleAsyncEvaluationCount]] to 0 whenever there are no
pending modules.
9.7 Agent Clusters
An agent cluster is a maximal set of agents that can communicate by
operating on shared memory.
Note 1
Programs within different agents may share memory by unspecified means. At a
minimum, the backing memory for SharedArrayBuffers can be shared among the agents in the cluster.
There may be agents
that can communicate by message passing that cannot share memory; they are never in the same
agent cluster.
The agents in a
cluster need not all be alive at some particular point in time. If agentA creates another agentB, after which
A terminates and B creates agentC, the three agents are in the same cluster if
A could share some memory with B and B could share some memory with
C.
All agents within a cluster
must have the same value for the [[LittleEndian]] field in their respective
Agent Records.
Note 3
If different agents
within an agent cluster have different values of [[LittleEndian]]
it becomes hard to use shared memory for multi-byte data.
All agents within a cluster
must have the same values for the [[IsLockFree1]] field in their respective
Agent Records;
similarly for the [[IsLockFree2]] field.
All agents within a cluster
must have different values for the [[Signifier]] field in their respective
Agent Records.
An embedding may deactivate (stop forward progress) or activate (resume forward progress) an
agent without the agent's knowledge or cooperation.
If the embedding does so, it must not leave some agents in the cluster active while other agents in the cluster are
deactivated indefinitely.
Note 4
The purpose of the preceding restriction is to avoid a situation where an agent deadlocks or starves
because another agent
has been deactivated. For example, if an HTML shared worker that has a lifetime independent
of documents in any windows were allowed to share memory with the dedicated worker of such
an independent document, and the document and its dedicated worker were to be deactivated
while the dedicated worker holds a lock (say, the document is pushed into its window's
history), and the shared worker then tries to acquire the lock, then the shared worker will
be blocked until the dedicated worker is activated again, if ever. Meanwhile other workers
trying to access the shared worker from other windows will starve.
The implication of the restriction is that it will not be possible to share memory between
agents that don't
belong to the same suspend/wake collective within the embedding.
An embedding may terminate an agent without any of the agent's cluster's other agents' prior knowledge or cooperation. If an
agent is terminated not by
programmatic action of its own or of another agent in the cluster but by forces external to the cluster,
then the embedding must choose one of two strategies: Either terminate all the agents in the cluster, or provide
reliable APIs that allow the agents in the cluster to coordinate so that at least one
remaining member of the cluster will be able to detect the termination, with the termination data
containing enough information to identify the agent that was terminated.
Note 5
Examples of that type of termination are: operating systems or users terminating agents that are running in
separate processes; the embedding itself terminating an agent that is running in-process with the
other agents when
per-agent resource
accounting indicates that the agent is runaway.
Each of the following specification values, and values transitively reachable from them, belong to
exactly one agent cluster.
An agent cluster is a specification mechanism and need not correspond to any particular
artefact of an ECMAScript implementation.
9.8 Forward Progress
For an agent to make
forward progress is for it to perform an evaluation step according to this specification.
An agent becomes
blocked when its running execution context waits
synchronously and indefinitely for an external event. Only agents whose Agent Record's [[CanBlock]]
field is true can become blocked in this sense. An unblockedagent is one that is not blocked.
Implementations must ensure that:
every unblocked agent
with a dedicated executing thread eventually makes forward
progress
an agent does not cause
another agent to become
blocked except via explicit APIs that provide blocking.
Note
This, along with the liveness guarantee in the memory model, ensures that all
seq-cst writes eventually become observable to all agents.
9.9 Processing Model of WeakRef and FinalizationRegistry Targets
9.9.1 Objectives
This specification does not make any guarantees that any object or symbol will be garbage
collected. Objects or symbols which are not live may be released after long periods of time, or
never at all. For this reason, this specification uses the term "may" when describing behaviour
triggered by garbage collection.
The semantics of WeakRefs and FinalizationRegistrys is
based on two operations which happen at particular points in time:
When WeakRef.prototype.deref is called, the referent (if
undefined is not returned) is kept alive so that subsequent, synchronous
accesses also return the same value. This list is reset when synchronous work is done using
the ClearKeptObjects abstract operation.
Some ECMAScript implementations include garbage collector implementations which run in the
background, including when ECMAScript is idle. Letting the host environment schedule
CleanupFinalizationRegistry
allows it to resume ECMAScript execution in order to run finalizer work, which may free up held
values, reducing overall memory usage.
9.9.2 Liveness
For some set of objects and/or symbols S a hypothetical
WeakRef-oblivious execution with respect to S is an execution whereby the
abstract operation WeakRefDeref of a WeakRef whose referent is an element of
S always returns undefined.
Note 1
WeakRef-obliviousness, together with
liveness, capture two notions. One, that a WeakRef itself does
not keep its referent alive. Two, that cycles in liveness does not imply that a value is
live. To be concrete, if determining v's liveness depends on determining the
liveness of a WeakRef referent, r,
r's liveness cannot assume v's liveness, which would be circular
reasoning.
Note 2
WeakRef-obliviousness is defined on
sets of objects or symbols instead of individual values to account for cycles. If it were
defined on individual values, then a WeakRef referent in a
cycle will be considered live even though its identity is only observed via other WeakRef referents in the cycle.
Note 3
Colloquially, we say that an individual object or symbol is live if every set containing it
is live.
At any point during evaluation, a set of objects and/or symbols S is considered live if either of the following conditions is met:
Any element in S is included in any agent's [[KeptAlive]]List.
There exists a valid future hypothetical WeakRef-oblivious execution with respect to
S that observes the identity of any value in S.
Note 4
The second condition above intends to capture the intuition that a value is live if its
identity is observable via non-WeakRef means. A value's identity
may be observed by observing a strict equality comparison or observing the value being used
as key in a Map.
Note 5
Presence of an object or a symbol in a field, an internal slot, or a property does not
imply that the value is live. For example if the value in question is never passed back
to the program, then it cannot be observed.
This is the case for keys in a WeakMap, members of a WeakSet, as well as the [[WeakRefTarget]] and [[UnregisterToken]] fields of a FinalizationRegistry
Cell record.
The above definition implies that, if a key in a WeakMap is not live, then its
corresponding value is not necessarily live either.
Note 6
Liveness is the lower bound for guaranteeing which WeakRefs engines must not empty.
Liveness as defined here is undecidable. In practice, engines use conservative
approximations such as reachability. There is expected to be significant implementation
leeway.
9.9.3 Execution
At any time, if a set of objects and/or symbols S is not live, an
ECMAScript implementation may perform the following steps atomically:
For each element value of S, do
For each WeakRefref
such that ref.[[WeakRefTarget]] is
value, do
Set ref.[[WeakRefTarget]] to
empty.
For each FinalizationRegistryfg such that fg.[[Cells]]
contains a Recordcell such that cell.[[WeakRefTarget]] is value, do
For each WeakMap map such that map.[[WeakMapData]] contains a Recordr such that r.[[Key]] is
value, do
Set r.[[Key]] to
empty.
Set r.[[Value]] to
empty.
For each WeakSet set such that set.[[WeakSetData]] contains value, do
Replace the element of set.[[WeakSetData]] whose value is value
with an element whose value is empty.
Note 1
Together with the definition of liveness, this clause prescribes optimizations that an
implementation may apply regarding WeakRefs.
It is possible to access an object without observing its identity. Optimizations such as
dead variable elimination and scalar replacement on properties of non-escaping objects
whose identity is not observed are allowed. These optimizations are thus allowed to
observably empty WeakRefs that point to such
objects.
On the other hand, if an object's identity is observable, and that object is in the [[WeakRefTarget]] internal slot of a WeakRef, optimizations such as
rematerialization that observably empty the WeakRef are
prohibited.
Implementations are not obligated to empty WeakRefs for
maximal sets of non-live objects or symbols.
If an implementation chooses a non-live set S in which to empty
WeakRefs, this definition
requires that it empties WeakRefs for all values in
S simultaneously. In other words, it is not conformant for an implementation
to empty a WeakRef pointing to a value
v without emptying out other WeakRefs that, if
not emptied, could result in an execution that observes the value of v.
The host-defined abstract operation
HostEnqueueFinalizationRegistryCleanupJob takes argument finalizationRegistry (a
FinalizationRegistry)
and returns unused.
Let cleanupJob be a new JobAbstract Closure with no
parameters that captures finalizationRegistry and performs the following steps
when called:
An implementation of HostEnqueueFinalizationRegistryCleanupJob schedules
cleanupJob to be performed at some future time, if possible. It must also conform
to the requirements in 9.5.
9.10 ClearKeptObjects ( )
The abstract operation ClearKeptObjects takes no arguments and returns unused.
ECMAScript implementations are expected to call ClearKeptObjects when a synchronous sequence of
ECMAScript executions completes. It performs the following steps when called:
When the abstract operation AddToKeptObjects is called with a target object or symbol, it adds
the target to a list that will point strongly at the target until ClearKeptObjects is called.
Assert:
finalizationRegistry has [[Cells]] and [[CleanupCallback]] internal slots.
Let callback be finalizationRegistry.[[CleanupCallback]].
While finalizationRegistry.[[Cells]] contains a
Recordcell
such that cell.[[WeakRefTarget]] is
empty, an implementation may perform the following steps:
The abstract operation CanBeHeldWeakly takes argument v (an ECMAScript language value) and returns
a Boolean. It returns true if and only if v is suitable for use as a
weak reference. Only values that are suitable for use as a weak reference may be a key of a WeakMap,
an element of a WeakSet, the target of a WeakRef, or one of the targets
of a FinalizationRegistry. It
performs the following steps when called:
A language value without language identity can be manifested without
prior reference and is unsuitable for use as a weak reference. A Symbol value produced by
Symbol.for, unlike other Symbol values, does
not have language identity and is unsuitable for use as a weak reference. Well-known symbols are likely to never
be collected, but are nonetheless treated as suitable for use as a weak reference because
they are limited in number and therefore manageable by a variety of implementation
approaches. However, any value associated to a well-known symbol in a live WeakMap
is unlikely to be collected and could “leak” memory resources in implementations.
10 Ordinary and Exotic Objects Behaviours
10.1 Ordinary Object Internal Methods and Internal Slots
All ordinary
objects have an internal slot called [[Prototype]].
The value of this internal slot is either null or an object and is used for
implementing inheritance. Assume a property named P is missing from an ordinary objectO but exists on its [[Prototype]] object. If P refers
to a data
property on the [[Prototype]] object, O
inherits it for get access, making it behave as if P was a property of O. If
P refers to a writable data property on the [[Prototype]] object, set access of P on O creates a
new data
property named P on O. If P refers to a
non-writable data
property on the [[Prototype]] object, set access of
P on O fails. If P refers to an accessor property on the [[Prototype]] object, the accessor is inherited by O for both get
access and set access.
Every ordinary
object has a Boolean-valued [[Extensible]] internal
slot which is used to fulfill the extensibility-related internal method invariants specified in
6.1.7.3. Namely,
once the value of an object's [[Extensible]] internal slot has been set to
false, it is no longer possible to add properties to the object, to modify the
value of the object's [[Prototype]] internal slot, or to subsequently
change the value of [[Extensible]] to true.
Each ordinary
object internal method delegates to a similarly-named abstract operation. If
such an abstract operation depends on another internal method, then the internal method is invoked
on O rather than calling the similarly-named abstract operation directly. These semantics
ensure that exotic
objects have their overridden internal methods invoked when ordinary object
internal methods are applied to them.
10.1.1[[GetPrototypeOf]] ( )
The [[GetPrototypeOf]] internal method of an ordinary
objectO takes no arguments and returns a normal completion
containing either an Object or null. It performs the
following steps when called:
The abstract operation OrdinaryGetPrototypeOf takes argument O (an Object) and
returns an Object or null. It performs the following steps when called:
Return O.[[Prototype]].
10.1.2[[SetPrototypeOf]] ( V )
The [[SetPrototypeOf]] internal method of an ordinary
objectO takes argument V (an Object or
null) and returns a normal completion
containing a Boolean. It performs the following steps when called:
The abstract operation OrdinarySetPrototypeOf takes arguments O (an Object) and
V (an Object or null) and returns a Boolean. It performs the
following steps when called:
If p.[[GetPrototypeOf]] is not
the ordinary object
internal method defined in 10.1.1,
set done to true.
Else, set p to p.[[Prototype]].
Set O.[[Prototype]] to V.
Return true.
Note
The loop in step 7 guarantees that
there will be no cycles in any prototype chain that only includes objects that use
the ordinary object definitions for [[GetPrototypeOf]] and [[SetPrototypeOf]].
10.1.3[[IsExtensible]] ( )
The [[IsExtensible]] internal method of an ordinary
objectO takes no arguments and returns a normal completion
containing a Boolean. It performs the following steps when called:
The abstract operation OrdinaryIsExtensible takes argument O (an Object) and
returns a Boolean. It performs the following steps when called:
Return O.[[Extensible]].
10.1.4[[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of an ordinary
objectO takes no arguments and returns a normal completion
containingtrue. It performs the following steps when
called:
The abstract operation OrdinaryGetOwnProperty takes arguments O (an Object) and
P (a property key) and returns a Property
Descriptor or undefined. It performs the following
steps when called:
If O does not have an own property with key P, return
undefined.
10.1.6.2 IsCompatiblePropertyDescriptor ( Extensible,
Desc, Current )
The abstract operation IsCompatiblePropertyDescriptor takes arguments Extensible
(a Boolean), Desc (a Property
Descriptor), and Current (a Property
Descriptor or undefined) and returns a Boolean. It
performs the following steps when called:
10.1.6.3 ValidateAndApplyPropertyDescriptor ( O,
P, extensible, Desc, current )
The abstract operation ValidateAndApplyPropertyDescriptor takes arguments O (an
Object or undefined), P (a property key),
extensible (a Boolean), Desc (a Property
Descriptor), and current (a Property
Descriptor or undefined) and returns a Boolean. It
returns true if and only if Desc can be applied as the
property of an object with specified extensibility and current property
current while upholding invariants.
When such application is possible and O is not undefined, it
is performed for the property named P (which is created if necessary). It
performs the following steps when called:
Create an own accessor property
named P of object O whose [[Get]], [[Set]],
[[Enumerable]], and [[Configurable]] attributes are set to the
value of the corresponding field in Desc if
Desc has that field, or to the attribute's default
value otherwise.
Else,
Create an own data property named
P of object O whose [[Value]], [[Writable]], [[Enumerable]], and [[Configurable]] attributes are set to the
value of the corresponding field in Desc if
Desc has that field, or to the attribute's default
value otherwise.
If Desc has a [[Get]] field and
SameValue(Desc.[[Get]], current.[[Get]]) is false, return
false.
If Desc has a [[Set]] field and
SameValue(Desc.[[Set]], current.[[Set]]) is false, return
false.
Else if current.[[Writable]] is
false, then
If Desc has a [[Writable]] field
and Desc.[[Writable]] is
true, return false.
NOTE: SameValue returns
true for NaN values which may
be distinguishable by other means. Returning here ensures that any
existing property of O remains unmodified.
If Desc has a [[Value]] field,
return SameValue(Desc.[[Value]], current.[[Value]]).
If Desc has a [[Configurable]]
field, let configurable be Desc.[[Configurable]]; else let
configurable be current.[[Configurable]].
If Desc has a [[Enumerable]]
field, let enumerable be Desc.[[Enumerable]]; else let
enumerable be current.[[Enumerable]].
Replace the property named P of object O with
an accessor property
whose [[Configurable]] and [[Enumerable]] attributes are set to
configurable and enumerable, respectively, and
whose [[Get]] and [[Set]] attributes are set to the value of
the corresponding field in Desc if Desc has
that field, or to the attribute's default
value otherwise.
If Desc has a [[Configurable]]
field, let configurable be Desc.[[Configurable]]; else let
configurable be current.[[Configurable]].
If Desc has a [[Enumerable]]
field, let enumerable be Desc.[[Enumerable]]; else let
enumerable be current.[[Enumerable]].
Replace the property named P of object O with
a data property whose
[[Configurable]] and [[Enumerable]] attributes are set to
configurable and enumerable, respectively, and
whose [[Value]] and [[Writable]] attributes are set to the value
of the corresponding field in Desc if Desc has
that field, or to the attribute's default
value otherwise.
Else,
For each field of Desc, set the corresponding attribute
of the property named P of object O to the
value of the field.
The abstract operation OrdinaryOwnPropertyKeys takes argument O (an Object) and
returns a List of property
keys. It performs the following steps when called:
For each own property keyP of
O such that P is an array index, in ascending
numeric index order, do
Append P to keys.
For each own property keyP of
O such that Pis a String
and P is not an array index, in ascending chronological
order of property creation, do
Append P to keys.
For each own property keyP of
O such that Pis a
Symbol, in ascending chronological order of property
creation, do
Append P to keys.
Return keys.
10.1.12 OrdinaryObjectCreate ( proto [ ,
additionalInternalSlotsList ] )
The abstract operation OrdinaryObjectCreate takes argument proto (an Object or
null) and optional argument additionalInternalSlotsList (a
List of names of internal
slots) and returns an Object. It is used to specify the runtime creation of new ordinary
objects. additionalInternalSlotsList contains the names of
additional internal slots that must be defined as part of the object, beyond [[Prototype]] and [[Extensible]]. If
additionalInternalSlotsList is not provided, a new empty List is used. It performs
the following steps when called:
Let internalSlotsList be « [[Prototype]], [[Extensible]] ».
If additionalInternalSlotsList is present, set internalSlotsList
to the list-concatenation of
internalSlotsList and additionalInternalSlotsList.
Although OrdinaryObjectCreate does little more than call MakeBasicObject, its
use communicates the intention to create an ordinary object, and not an
exotic one. Thus, within this specification, it is not called by any algorithm that
subsequently modifies the internal methods of the object in ways that would make the
result non-ordinary. Operations that create exotic objects invoke
MakeBasicObject directly.
The abstract operation OrdinaryCreateFromConstructor takes arguments constructor (a
function
object) and intrinsicDefaultProto (a String) and optional
argument internalSlotsList (a List of names of internal
slots) and returns either a normal completion
containing an Object or a throw completion. It
creates an ordinary
object whose [[Prototype]] value is retrieved
from a constructor's "prototype"
property, if it exists. Otherwise the intrinsic named by intrinsicDefaultProto is
used for [[Prototype]]. internalSlotsList contains the names
of additional internal slots that must be defined as part of the object. If
internalSlotsList is not provided, a new empty List is used. It performs
the following steps when called:
Assert:
intrinsicDefaultProto is this specification's name of an intrinsic object.
The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
The abstract operation GetPrototypeFromConstructor takes arguments constructor (a
function
object) and intrinsicDefaultProto (a String) and returns
either a normal completion
containing an Object or a throw completion. It
determines the [[Prototype]] value that should be used to create an
object corresponding to a specific constructor. The value is retrieved from the
constructor's
"prototype" property, if it exists. Otherwise the intrinsic named by
intrinsicDefaultProto is used for [[Prototype]]. It performs
the following steps when called:
Assert:
intrinsicDefaultProto is this specification's name of an intrinsic object.
The corresponding object must be an intrinsic that is intended to be used as the [[Prototype]] value of an object.
Set proto to realm's intrinsic object named
intrinsicDefaultProto.
Return proto.
Note
If constructor does not supply a [[Prototype]] value,
the default value that is used is obtained from the realm of the constructor
function rather than from the running execution
context.
10.1.15 RequireInternalSlot ( O, internalSlot
)
The abstract operation RequireInternalSlot takes arguments O (an ECMAScript language value) and
internalSlot (an internal slot name) and returns either a normal completion
containingunused or a throw completion. It
throws an exception unless Ois an Object and has the given internal slot. It
performs the following steps when called:
The PrivateEnvironment
Record for Private Names
that the function was closed over. null if this function is
not syntactically contained within a class. Used as the outer PrivateEnvironment
for inner classes when evaluating the code of the function.
The script or module in which the function was created.
[[ThisMode]]
lexical, strict, or
global
Defines how this references are interpreted within the formal
parameters and code body of the function. lexical means
that this refers to the this value of a
lexically enclosing function. strict means that the
this value is used exactly as provided by an invocation of
the function. global means that a this
value of undefined or null is interpreted
as a reference to the global object, and any other
this value is first passed to ToObject.
If the function is created as the initializer of a class field, the name to use
for NamedEvaluation
of the field; empty otherwise.
[[IsClassConstructor]]
a Boolean
Indicates whether the function is a class constructor. (If
true, invoking the function's [[Call]] will immediately throw a
TypeError exception.)
All ECMAScript function
objects have the [[Call]] internal method defined
here. ECMAScript functions that are also constructors in addition have the [[Construct]] internal method.
When calleeContext is removed from the execution context stack in step
7 it must not be destroyed if
it is suspended and retained for later resumption by an accessible Generator.
10.2.1.1 PrepareForOrdinaryCall ( F,
newTarget )
The abstract operation PrepareForOrdinaryCall takes arguments F (an ECMAScript
function
object) and newTarget (an Object or
undefined) and returns an execution context. It
performs the following steps when called:
The abstract operation OrdinaryCallBindThis takes arguments F (an ECMAScript
function
object), calleeContext (an execution context), and
thisArgument (an ECMAScript language
value) and returns unused. It performs the
following steps when called:
Let thisMode be F.[[ThisMode]].
If thisMode is lexical, return
unused.
Let calleeRealm be F.[[Realm]].
Let localEnv be the LexicalEnvironment of calleeContext.
Even though field initializers constitute a function boundary, calling FunctionDeclarationInstantiation
does not have any observable effect and so is omitted.
The abstract operation OrdinaryFunctionCreate takes arguments functionPrototype (an
Object), sourceText (a sequence of Unicode code points), ParameterList (a
Parse
Node), Body (a Parse Node),
thisMode (lexical-this or
non-lexical-this), env (an Environment Record), and
privateEnv (a PrivateEnvironment Record or
null) and returns an ECMAScript function object. It is used to
specify the runtime creation of a new function with a default [[Call]]
internal method and no [[Construct]] internal method (although one may
be subsequently added by an operation such as MakeConstructor). sourceText is
the source text of the syntactic definition of the function to be created. It performs the
following steps when called:
Let internalSlotsList be the internal slots listed in Table
30.
The abstract operation AddRestrictedFunctionProperties takes arguments F (a function
object) and realm (a Realm Record) and returns
unused. It performs the following steps when called:
The abstract operation MakeConstructor takes argument F (an ECMAScript function
object or a built-in function object) and optional arguments
writablePrototype (a Boolean) and prototype (an Object) and returns
unused. It converts F into a constructor. It performs the following
steps when called:
The abstract operation MakeClassConstructor takes argument F (an ECMAScript function
object) and returns unused. It performs the
following steps when called:
The abstract operation MakeMethod takes arguments F (an ECMAScript function
object) and homeObject (an Object) and returns
unused. It configures F as a method. It performs the following
steps when called:
The abstract operation SetFunctionName takes arguments F (a function
object) and name (a property key or Private
Name) and optional argument prefix (a String) and returns
unused. It adds a "name" property to F. It
performs the following steps when called:
Assert:
F is an extensible object that does not have a "name" own
property.
The abstract operation SetFunctionLength takes arguments F (a function
object) and length (a non-negative integer or +∞) and returns
unused. It adds a "length" property to F.
It performs the following steps when called:
Assert:
F is an extensible object that does not have a "length"
own property.
When an execution context is established
for evaluating an ECMAScript function a new Function Environment
Record is created and bindings for each formal parameter are
instantiated in that Environment Record. Each
declaration in the function body is also instantiated. If the function's formal
parameters do not include any default value initializers then the body declarations are
instantiated in the same Environment Record as the
parameters. If default value parameter initializers exist, a second Environment Record is created for
the body declarations. Formal parameters and functions are initialized as part of
FunctionDeclarationInstantiation. All other bindings are initialized during evaluation
of the function body.
Let fn be the sole element of the BoundNames
of d.
If functionNames does not contain fn, then
Insert fn as the first element of
functionNames.
NOTE: If there are multiple function declarations for the same
name, the last declaration is used.
Insert d as the first element of
functionsToInitialize.
Let argumentsObjectNeeded be true.
If func.[[ThisMode]] is
lexical, then
NOTE: Arrow functions never have an arguments object.
Set argumentsObjectNeeded to false.
Else if parameterNames contains "arguments", then
Set argumentsObjectNeeded to false.
Else if hasParameterExpressions is false, then
If functionNames contains "arguments" or
lexicalNames contains "arguments", then
Set argumentsObjectNeeded to false.
If strict is true or hasParameterExpressions is
false, then
NOTE: Only a single Environment Record is
needed for the parameters, since calls to eval in strict mode code cannot
create new bindings which are visible outside of the eval.
Let env be the LexicalEnvironment of calleeContext.
Else,
NOTE: A separate Environment Record is
needed to ensure that bindings created by direct
eval calls in the formal parameter list are outside the
environment where parameters are declared.
Let calleeEnv be the LexicalEnvironment of calleeContext.
Set the LexicalEnvironment of calleeContext to env.
For each String paramName of parameterNames, do
Let alreadyDeclared be
! env.HasBinding(paramName).
NOTE: Early
errors ensure that duplicate parameter names can only
occur in non-strict functions that do
not have parameter default values or rest parameters.
NOTE: A mapped argument object is only provided for non-strict functions
that don't have a rest parameter, any parameter default value
initializers, or any destructured parameters.
NOTE: The following step cannot return a ReturnCompletion
because the only way such a completion can arise in expression position is by use of
YieldExpression,
which is forbidden in parameter lists by Early Error rules in 15.5.1
and 15.6.1.
NOTE: Only a single Environment Record is
needed for the parameters and top-level vars.
Let instantiatedVarNames be a copy of the ListparameterBindings.
For each element n of varNames, do
If instantiatedVarNames does not contain n, then
Append n to instantiatedVarNames.
Perform ! env.CreateMutableBinding(n,
false).
Perform ! env.InitializeBinding(n,
undefined).
Let varEnv be env.
Else,
NOTE: A separate Environment Record is
needed to ensure that closures created by expressions in the formal parameter
list do not have visibility of declarations in the function body.
NOTE: Non-strict functions use a
separate Environment Record for
top-level lexical declarations so that a direct
eval can determine whether any var scoped declarations
introduced by the eval code conflict with pre-existing top-level lexically
scoped declarations. This is not needed for strict functions
because a strict direct
eval always places all declarations into a new Environment Record.
Else,
Let lexEnv be varEnv.
Set the LexicalEnvironment of calleeContext to lexEnv.
NOTE: A lexically declared name cannot be the same as a function/generator
declaration, formal parameter, or a var name. Lexically declared names are only
instantiated here but not initialized.
B.3.2
provides an extension to the above algorithm that is necessary for backwards
compatibility with web browser implementations of ECMAScript that predate ECMAScript
2015.
In addition to the internal slots required of every ordinary object (see 10.1), a
built-in function
object must also have the following internal slots:
[[Realm]], a Realm Record that represents the realm in which the function was
created.
[[InitialName]], a String that is the initial name of the function. It
is used by 20.2.3.5.
A built-in function
object must have a [[Call]] internal method that
conforms to the definition in 10.3.1.
A built-in function
object has a [[Construct]] internal method if and
only if it is described as a “constructor”, or some algorithm in this specification
explicitly sets its [[Construct]] internal method. Such a [[Construct]] internal method must conform to the definition in 10.3.2.
An implementation may provide additional built-in function objects that are not defined in this
specification.
Let result be the Completion
Record that is the result of
evaluatingF in a manner that conforms to the specification of
F. If thisArgument is uninitialized, the
this value is uninitialized; otherwise, thisArgument
provides the this value. argumentsList provides the named
parameters. newTarget provides the NewTarget value.
NOTE: If F is defined in this document, “the specification of F”
is the behaviour specified for it via algorithm steps or other means.
When calleeContext is removed from the execution context stack it must
not be destroyed if it has been suspended and retained by an accessible Generator for
later resumption.
The abstract operation CreateBuiltinFunction takes arguments behaviour (an Abstract
Closure, a set of algorithm steps, or some other definition of a
function's behaviour provided in this specification), length (a non-negative
integer or +∞),
name (a property key or a Private
Name), and additionalInternalSlotsList (a List of names of internal
slots) and optional arguments realm (a Realm Record), prototype
(an Object or null), and prefix (a String) and returns a built-in
function
object. additionalInternalSlotsList contains the names of
additional internal slots that must be defined as part of the object. This operation creates a
built-in function
object. It performs the following steps when called:
If prototype is not present, set prototype to
realm.[[Intrinsics]].[[%Function.prototype%]].
Let internalSlotsList be a List containing
the names of all the internal slots that 10.3 requires
for the built-in function object that is about to be
created.
Append to internalSlotsList the elements of
additionalInternalSlotsList.
Let func be a new built-in function object that, when
called, performs the action described by behaviour using the provided
arguments as the values of the corresponding parameters specified by
behaviour. The new function object has internal slots whose
names are the elements of internalSlotsList, and an [[InitialName]] internal slot.
Each built-in function defined in this specification is created by calling the
CreateBuiltinFunction abstract operation.
10.4 Built-in Exotic Object Internal Methods and Slots
This specification defines several kinds of built-in exotic objects. These objects generally
behave similar to ordinary objects except for a few specific
situations. The following exotic objects use the ordinary object
internal methods except where it is explicitly specified otherwise below:
An object is a bound function exotic object if its [[Call]]
and (if applicable) [[Construct]] internal methods use the following
implementations, and its other essential internal methods use the definitions found in 10.1.
These methods are installed in BoundFunctionCreate.
Set obj.[[BoundTargetFunction]] to
targetFunction.
Set obj.[[BoundThis]] to boundThis.
Set obj.[[BoundArguments]] to
boundArgs.
Return obj.
10.4.2 Array Exotic Objects
An Array is an exotic
object that gives special treatment to array indexproperty keys
(see 6.1.7). A property whose property name
is an array
index is also called an element. Every Array has a
non-configurable "length" property whose value is always a non-negative
integral
Number whose mathematical value is strictly less than
232. The value of the "length" property is numerically greater
than the name of every own property whose name is an array index; whenever an own property
of an Array is created or changed, other properties are adjusted as necessary to maintain this
invariant. Specifically, whenever an own property is added whose name is an array index, the
value of the "length" property is changed, if necessary, to be one more than
the numeric value of that array index; and whenever the value of the
"length" property is changed, every own property whose name is an array index whose
value is not smaller than the new length is deleted. This constraint applies only to own
properties of an Array and is unaffected by "length" or array index
properties that may be inherited from its prototypes.
An object is an Array
exotic object (or simply, an Array) if its [[DefineOwnProperty]] internal method uses the following implementation,
and its other essential internal methods use the definitions found in 10.1.
These methods are installed in ArrayCreate.
The abstract operation ArrayCreate takes argument length (a non-negative integer) and optional
argument proto (an Object) and returns either a normal completion
containing an Array exotic object or a throw completion. It
is used to specify the creation of new Arrays. It performs the following steps when called:
If length > 232 - 1, throw a RangeError
exception.
The abstract operation ArraySpeciesCreate takes arguments originalArray (an
Object) and length (a non-negative integer) and returns either a normal completion
containing an Object or a throw completion. It
is used to specify the creation of a new Array or similar object using a constructor
function that is derived from originalArray. It does not enforce that the
constructor function returns an Array. It
performs the following steps when called:
If originalArray was created using the standard built-in Array constructor for a realm that is not the realm of the
running execution context,
then a new Array is created using the realm of the running execution context.
This maintains compatibility with Web browsers that have historically had that
behaviour for the Array.prototype methods that now are defined using
ArraySpeciesCreate.
In steps 3 and 4, if
Desc.[[Value]] is an object then its
valueOf method is called twice. This is legacy behaviour that was
specified with this effect starting with the 2nd Edition of this
specification.
10.4.3 String Exotic Objects
A String object is an exotic object that encapsulates a String value and
exposes virtual integer-indexeddata properties corresponding to
the individual code unit elements of the String value. String exotic objects always
have a data
property named "length" whose value is the length of
the encapsulated String value. Both the code unit data properties and the
"length" property are non-writable and non-configurable.
An object is a String exotic object (or simply, a String object) if its [[GetOwnProperty]], [[DefineOwnProperty]], and
[[OwnPropertyKeys]] internal methods use the following implementations,
and its other essential internal methods use the definitions found in 10.1.
These methods are installed in StringCreate.
For each own property keyP of
O such that Pis a String
and P is not an array index, in ascending chronological
order of property creation, do
Append P to keys.
For each own property keyP of
O such that Pis a
Symbol, in ascending chronological order of property
creation, do
Append P to keys.
Return keys.
10.4.3.4 StringCreate ( value, prototype )
The abstract operation StringCreate takes arguments value (a String) and
prototype (an Object) and returns a String exotic object. It
is used to specify the creation of new String exotic objects. It
performs the following steps when called:
Let S be MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
Set S.[[Prototype]] to prototype.
Set S.[[StringData]] to value.
Set S.[[GetOwnProperty]] as specified in
10.4.3.1.
Set S.[[DefineOwnProperty]] as specified in
10.4.3.2.
Set S.[[OwnPropertyKeys]] as specified in
10.4.3.3.
The abstract operation StringGetOwnProperty takes arguments S (an Object that has
a [[StringData]] internal slot) and P (a property key)
and returns a Property
Descriptor or undefined. It performs the following
steps when called:
Most ECMAScript functions make an arguments object available to their code. Depending upon the
characteristics of the function definition, its arguments object is either an ordinary
object or an arguments exotic object. An arguments
exotic object is an exotic object whose array index properties map to the
formal parameters bindings of an invocation of its associated ECMAScript function.
An object is an arguments exotic object if its internal methods use the following
implementations, with the ones not specified here using those found in 10.1.
These methods are installed in CreateMappedArgumentsObject.
Arguments exotic objects have the same
internal slots as ordinary objects. They also have a [[ParameterMap]] internal slot. Ordinary arguments objects also have a
[[ParameterMap]] internal slot whose value is always
undefined. For ordinary argument objects the [[ParameterMap]] internal slot is only used by
Object.prototype.toString (20.1.3.6) to identify
them as such.
Note 2
The integer-indexeddata
properties of an arguments exotic
object whose numeric name values are less than the number of
formal parameters of the corresponding function object initially
share their values with the corresponding argument bindings in the function's execution context. This means that
changing the property changes the corresponding value of the argument binding and
vice-versa. This correspondence is broken if such a property is deleted and then
redefined or if the property is changed into an accessor property. If the
arguments object is an ordinary object, the values of its
properties are simply a copy of the arguments passed to the function and there is no
dynamic linkage between the property values and the formal parameter values.
Note 3
The ParameterMap object and its property values are used as a device for specifying the
arguments object correspondence to argument bindings. The ParameterMap object and the
objects that are the values of its properties are not directly observable from
ECMAScript code. An ECMAScript implementation does not need to actually create or use
such objects to implement the specified semantics.
Note 4
Ordinary arguments objects define a non-configurable accessor property named
"callee" which throws a TypeError exception on
access. The "callee" property has a more specific meaning for
arguments exotic objects, which
are created only for some class of non-strict functions.
The definition of this property in the ordinary variant exists to ensure that it is not
defined in any other manner by conforming ECMAScript implementations.
Note 5
ECMAScript implementations of arguments exotic objects have
historically contained an accessor property named
"caller". Prior to ECMAScript 2017, this specification included the
definition of a throwing "caller" property on ordinary arguments
objects. Since implementations do not contain this extension any longer, ECMAScript 2017
dropped the requirement for a throwing "caller" accessor.
The abstract operation CreateUnmappedArgumentsObject takes argument argumentsList
(a List of ECMAScript language values) and
returns an ordinary object. It performs the following
steps when called:
Let len be the number of elements in argumentsList.
The abstract operation MakeArgGetter takes arguments name (a String) and
env (an Environment Record) and returns a
function
object. It creates a built-in function object that when
executed returns the value bound for name in env. It performs the
following steps when called:
Let getterClosure be a new Abstract
Closure with no parameters that captures name
and env and performs the following steps when called:
NOTE: getter is never directly accessible to ECMAScript code.
Return getter.
10.4.4.7.2 MakeArgSetter ( name, env )
The abstract operation MakeArgSetter takes arguments name (a String) and
env (an Environment Record) and returns a
function
object. It creates a built-in function object that when
executed sets the value bound for name in env. It performs the
following steps when called:
Let setterClosure be a new Abstract
Closure with parameters (value) that captures
name and env and performs the following steps when called:
NOTE: setter is never directly accessible to ECMAScript code.
Return setter.
10.4.5 TypedArray Exotic Objects
A TypedArray is
an exotic
object that performs special handling of property keys that are canonical numeric strings, using
the subset that are in-bounds integer indices to index elements of uniform type
and enforcing the invariant that the remainder are absent without incurring prototype chain
traversal.
TypedArrays have
the same internal slots as ordinary objects and additionally [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]] internal
slots.
An object is a TypedArray if its [[PreventExtensions]], [[GetOwnProperty]], [[HasProperty]], [[DefineOwnProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]], internal
methods use the definitions in this section, and its other essential internal methods use the
definitions found in 10.1.
These methods are installed by TypedArrayCreate.
10.4.5.1[[PreventExtensions]] ( )
The [[PreventExtensions]] internal method of a TypedArrayO takes no arguments and returns a normal completion
containing a Boolean. It performs the following steps when called:
NOTE: The extensibility-related invariants specified in 6.1.7.3
do not allow this method to return true when O can
gain (or lose and then regain) properties, which might occur for properties with
integer
index names when its underlying buffer is resized.
For each own property keyP of
O such that Pis a
Symbol, in ascending chronological order of property
creation, do
Append P to keys.
Return keys.
10.4.5.9 TypedArray With Buffer Witness Records
An TypedArray With
Buffer Witness Record is a Record value used to
encapsulate a TypedArray along with a cached byte length of the
viewed buffer. It is used to help ensure there is a single shared memory read event of the
byte length data block when the viewed buffer is a growable
SharedArrayBuffer.
TypedArray With Buffer Witness Records have the fields listed in Table 32.
The byte length of the object's [[ViewedArrayBuffer]] when the Record
was created.
10.4.5.10 MakeTypedArrayWithBufferWitnessRecord (
obj, order )
The abstract operation MakeTypedArrayWithBufferWitnessRecord takes arguments obj
(a TypedArray) and order
(seq-cst or unordered) and returns a TypedArray With Buffer Witness
Record. It performs the following steps when called:
The abstract operation TypedArrayCreate takes argument prototype (an Object) and
returns a TypedArray. It is used to specify the creation of
new TypedArrays. It performs the following steps when
called:
Let internalSlotsList be « [[Prototype]], [[Extensible]], [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]],
[[ByteOffset]], [[ArrayLength]] ».
Set A.[[OwnPropertyKeys]] as specified in
10.4.5.8.
Set A.[[Prototype]] to prototype.
Return A.
10.4.5.12 TypedArrayByteLength ( taRecord )
The abstract operation TypedArrayByteLength takes argument taRecord (a TypedArray With Buffer Witness
Record) and returns a non-negative integer. It performs the following
steps when called:
The abstract operation TypedArrayLength takes argument taRecord (a TypedArray With Buffer Witness
Record) and returns a non-negative integer. It performs the following
steps when called:
The abstract operation IsTypedArrayOutOfBounds takes argument taRecord (a
TypedArray With Buffer Witness
Record) and returns a Boolean. It checks if any of the object's
numeric properties reference a value at an index not contained within the underlying
buffer's bounds. It performs the following steps when called:
Let O be taRecord.[[Object]].
Let bufferByteLength be taRecord.[[CachedBufferByteLength]].
Assert:
IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true if and
only if bufferByteLength is detached.
The abstract operation IsValidIntegerIndex takes arguments O (a TypedArray) and
index (a Number) and returns a Boolean. It performs the following steps when
called:
If IsDetachedBuffer(O.[[ViewedArrayBuffer]]) is true, return
false.
The abstract operation TypedArrayGetElement takes arguments O (a TypedArray) and
index (a Number) and returns a Number, a BigInt, or undefined.
It performs the following steps when called:
This operation always appears to succeed, but it has no effect when attempting to
write past the end of a TypedArray or to a TypedArray which is backed by a detached
ArrayBuffer.
10.4.5.19 IsArrayBufferViewOutOfBounds ( O )
The abstract operation IsArrayBufferViewOutOfBounds takes argument O (a TypedArray or a
DataView) and returns a Boolean. It checks if either any of a TypedArray's numeric properties or a
DataView object's methods can reference a value at an index not contained within the
underlying data block's bounds. This abstract operation exists as a convenience for upstream
specifications. It performs the following steps when called:
A module namespace exotic object is
an exotic
object that exposes the bindings exported from an ECMAScript Module (See 16.2.3). There is a
one-to-one correspondence between the String-keyed own properties of a module namespace exotic object and
the binding names exported by the Module. The exported bindings include any bindings that
are indirectly exported using export * export items. Each String-valued own
property
key is the StringValue of the corresponding
exported binding name. These are the only String-keyed properties of a module namespace exotic object.
Each such property has the attributes { [[Writable]]:
true, [[Enumerable]]: true, [[Configurable]]: false }. Module namespace exotic objects
are not extensible.
An object is a module namespace exotic
object if its [[GetPrototypeOf]], [[SetPrototypeOf]], [[IsExtensible]], [[PreventExtensions]], [[GetOwnProperty]], [[DefineOwnProperty]], [[HasProperty]], [[Get]], [[Set]], [[Delete]], and [[OwnPropertyKeys]] internal
methods use the definitions in this section, and its other essential internal methods use the
definitions found in 10.1.
These methods are installed by ModuleNamespaceCreate.
A List
whose elements are the String values of the exported names exposed as own
properties of this object. The list is sorted according to lexicographic code unit
order.
ResolveExport is side-effect free. Each time this operation is called with a specific
exportName, resolveSet pair as arguments it must return the
same result. An implementation might choose to pre-compute or cache the
ResolveExport results for the [[Exports]] of each module namespace exotic
object.
An object is an immutable prototype exotic
object if its [[SetPrototypeOf]] internal method uses the
following implementation. (Its other essential internal methods may use any implementation,
depending on the specific immutable prototype exotic
object in question.)
The abstract operation SetImmutablePrototype takes arguments O (an Object) and
V (an Object or null) and returns either a normal completion
containing a Boolean or a throw completion. It
performs the following steps when called:
10.5 Proxy Object Internal Methods and Internal Slots
A Proxy object is an exotic
object whose essential internal methods are partially implemented using
ECMAScript code. Every Proxy object has an internal slot called [[ProxyHandler]]. The value of [[ProxyHandler]] is
an object, called the proxy's handler object, or null. Methods (see
Table 34) of a handler object may be used
to augment the implementation for one or more of the Proxy object's internal methods. Every Proxy
object also has an internal slot called [[ProxyTarget]] whose value is
either an object or null. This object is called the proxy's target
object.
An object is a Proxy
exotic object if its essential internal methods (including [[Call]] and [[Construct]], if applicable) use the
definitions in this section. These internal methods are installed in ProxyCreate.
Table 34: Proxy Handler Methods
Internal Method
Handler Method
[[GetPrototypeOf]]
getPrototypeOf
[[SetPrototypeOf]]
setPrototypeOf
[[IsExtensible]]
isExtensible
[[PreventExtensions]]
preventExtensions
[[GetOwnProperty]]
getOwnPropertyDescriptor
[[DefineOwnProperty]]
defineProperty
[[HasProperty]]
has
[[Get]]
get
[[Set]]
set
[[Delete]]
deleteProperty
[[OwnPropertyKeys]]
ownKeys
[[Call]]
apply
[[Construct]]
construct
When a handler method is called to provide the implementation of a Proxy object internal method, the
handler method is passed the proxy's target object as a parameter. A proxy's handler object does not
necessarily have a method corresponding to every essential internal method. Invoking an internal
method on the proxy results in the invocation of the corresponding internal method on the proxy's
target object if the handler object does not have a method corresponding to the internal trap.
The [[ProxyHandler]] and [[ProxyTarget]] internal
slots of a Proxy object are always initialized when the object is created and typically may not be
modified. Some Proxy objects are created in a manner that permits them to be subsequently
revoked. When a proxy is revoked, its [[ProxyHandler]] and [[ProxyTarget]] internal slots are set to null causing
subsequent invocations of internal methods on that Proxy object to throw a
TypeError exception.
Because Proxy objects permit the implementation of internal methods to be provided by arbitrary
ECMAScript code, it is possible to define a Proxy object whose handler methods violates the
invariants defined in 6.1.7.3. Some of the
internal method invariants defined in 6.1.7.3 are
essential integrity invariants. These invariants are explicitly enforced by the Proxy object
internal methods specified in this section. An ECMAScript implementation must be robust in the
presence of all possible invariant violations.
If SameValue(handlerProto,
targetProto) is false, throw a
TypeError exception.
Return handlerProto.
Note
[[GetPrototypeOf]] for Proxy objects enforces the following
invariants:
The result of [[GetPrototypeOf]] must be either an Object
or null.
If the target object is not extensible, [[GetPrototypeOf]]
applied to the Proxy object must return the same value as [[GetPrototypeOf]] applied to the Proxy object's target
object.
[[IsExtensible]] applied to the Proxy object must return
the same value as [[IsExtensible]] applied to the Proxy
object's target object with the same argument.
If targetDesc.[[Writable]] is
true, throw a TypeError exception.
Return resultDesc.
Note
[[GetOwnProperty]] for Proxy objects enforces the following
invariants:
The result of [[GetOwnProperty]] must be either an Object
or undefined.
A property cannot be reported as non-existent, if it exists as a non-configurable
own property of the target object.
A property cannot be reported as non-existent, if it exists as an own property of a
non-extensible target object.
A property cannot be reported as existent, if it does not exist as an own property
of the target object and the target object is not extensible.
A property cannot be reported as non-configurable, unless it exists as a
non-configurable own property of the target object.
A property cannot be reported as both non-configurable and non-writable, unless it
exists as a non-configurable, non-writable own property of the target object.
If settingConfigFalse is true and
targetDesc.[[Configurable]] is
true, throw a TypeError exception.
If IsDataDescriptor(targetDesc)
is true, targetDesc.[[Configurable]] is false, and
targetDesc.[[Writable]] is
true, then
If Desc has a [[Writable]] field and
Desc.[[Writable]] is
false, throw a TypeError
exception.
Return true.
Note
[[DefineOwnProperty]] for Proxy objects enforces the following
invariants:
The result of [[DefineOwnProperty]]is a
Boolean value.
A property cannot be added, if the target object is not extensible.
A property cannot be non-configurable, unless there exists a corresponding
non-configurable own property of the target object.
A non-configurable property cannot be non-writable, unless there exists a
corresponding non-configurable, non-writable own property of the target object.
If a property has a corresponding target object property then applying the Property
Descriptor of the property to the target object using [[DefineOwnProperty]] will not throw an exception.
Let trapResult be ? Call(trap,
handler, « target, P, Receiver »).
Let targetDesc be ? target.[[GetOwnProperty]](P).
If targetDesc is not undefined and
targetDesc.[[Configurable]] is
false, then
If IsDataDescriptor(targetDesc)
is true and targetDesc.[[Writable]] is false, then
If SameValue(trapResult,
targetDesc.[[Value]]) is
false, throw a TypeError
exception.
If IsAccessorDescriptor(targetDesc)
is true and targetDesc.[[Get]] is undefined, then
If trapResult is not undefined, throw a
TypeError exception.
Return trapResult.
Note
[[Get]] for Proxy objects enforces the following invariants:
The value reported for a property must be the same as the value of the corresponding
target object property if the target object property is a non-writable,
non-configurable own data property.
The value reported for a property must be undefined if the
corresponding target object property is a non-configurable own accessor
property that has undefined as its [[Get]] attribute.
Cannot change the value of a property to be different from the value of the
corresponding target object property if the corresponding target object property is
a non-writable, non-configurable own data property.
Cannot set the value of a property if the corresponding target object property is a
non-configurable own accessor property that has
undefined as its [[Set]] attribute.
A Proxy exotic object only has a [[Call]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Call]] internal method.
A Proxy exotic object only has a [[Construct]] internal method if the initial value of its [[ProxyTarget]] internal slot is an object that has a [[Construct]] internal method.
Note 2
[[Construct]] for Proxy objects enforces the following
invariants:
The result of [[Construct]] must be an Object.
10.5.14 ValidateNonRevokedProxy ( proxy )
The abstract operation ValidateNonRevokedProxy takes argument proxy (a Proxy exotic
object) and returns either a normal completion
containingunused or a throw completion. It
throws a TypeError exception if proxy has been revoked. It
performs the following steps when called:
If proxy.[[ProxyTarget]] is null,
throw a TypeError exception.
ECMAScript source text is a sequence of Unicode code points. All Unicode
code point values from U+0000 to U+10FFFF, including surrogate code points, may occur in ECMAScript
source text where permitted by the ECMAScript grammars. The actual encodings used to store and
interchange ECMAScript source text is not relevant to this specification. Regardless of the external
source text encoding, a conforming ECMAScript implementation processes the source text as if it was
an equivalent sequence of SourceCharacter values, each SourceCharacter being a Unicode
code point. Conforming ECMAScript implementations are not required to perform any normalization of
source text, or behave as though they were performing normalization of source text.
The components of a combining character sequence are treated as individual Unicode code points even
though a user might think of the whole sequence as a single character.
Note
In string literals, regular expression literals, template literals and identifiers, any
Unicode code point may also be expressed using Unicode escape sequences that explicitly
express a code point's numeric value. Within a comment, such an escape sequence is
effectively ignored as part of the comment.
ECMAScript differs from the Java programming language in the behaviour of Unicode escape
sequences. In a Java program, if the Unicode escape sequence \u000A, for
example, occurs within a single-line comment, it is interpreted as a line terminator
(Unicode code point U+000A is LINE FEED (LF)) and therefore the next code point is not part
of the comment. Similarly, if the Unicode escape sequence \u000A occurs within
a string literal in a Java program, it is likewise interpreted as a line terminator, which
is not allowed within a string literal—one must write \n instead of
\u000A to cause a LINE FEED (LF) to be part of the value of a string literal.
In an ECMAScript program, a Unicode escape sequence occurring within a comment is never
interpreted and therefore cannot contribute to termination of the comment. Similarly, a
Unicode escape sequence occurring within a string literal in an ECMAScript program always
contributes to the literal and is never interpreted as a line terminator or as a code point
that might terminate the string literal.
The abstract operation UTF16EncodeCodePoint takes argument cp (a Unicode code point)
and returns a String. It performs the following steps when called:
11.1.2 Static Semantics: CodePointsToString ( text )
The abstract operation CodePointsToString takes argument text (a sequence of Unicode
code points) and returns a String. It converts text into a String value, as described
in 6.1.4. It performs the
following steps when called:
The abstract operation UTF16SurrogatePairToCodePoint takes arguments lead (a code
unit) and trail (a code unit) and returns a code point. Two code units that form a
UTF-16 surrogate
pair are converted to a code point. It performs the following steps when
called:
Let cp be (lead - 0xD800) × 0x400 + (trail - 0xDC00) +
0x10000.
Return the code point cp.
11.1.4 Static Semantics: CodePointAt ( string,
position )
The abstract operation CodePointAt takes arguments string (a String) and
position (a non-negative integer) and returns a Record with fields [[CodePoint]] (a code point), [[CodeUnitCount]]
(a positive integer),
and [[IsUnpairedSurrogate]] (a Boolean). It interprets
string as a sequence of UTF-16 encoded code points, as described in 6.1.4, and reads from
it a single code point starting with the code unit at index position. It performs the
following steps when called:
The abstract operation StringToCodePoints takes argument string (a String) and returns
a List of code points. It
returns the sequence of Unicode code points that results from interpreting string as
UTF-16 encoded Unicode text as described in 6.1.4. It performs the
following steps when called:
The abstract operation ParseText takes arguments sourceText (a String or a sequence of
Unicode code points) and goalSymbol (a nonterminal in one of the ECMAScript grammars)
and returns a Parse Node or a non-empty List of
SyntaxError objects. It performs the following steps when called:
If the parse succeeded and no early errors were found, return the Parse
Node (an instance of goalSymbol) at the root of the
parse tree resulting from the parse.
Otherwise, return a List
of one or more SyntaxError objects representing the parsing errors
and/or early
errors. If more than one parsing error or early error
is present, the number and ordering of error objects in the list is implementation-defined, but at
least one must be present.
Note 1
Consider a text that has an early error at a particular point, and also a
syntax error at a later point. An implementation that does a parse pass followed by an
early
errors pass might report the syntax error and not proceed to the
early
errors pass. An implementation that interleaves the two
activities might report the early error and not proceed to find the
syntax error. A third implementation might report both errors. All of these behaviours
are conformant.
Eval code is the source text supplied to the built-in eval
function. More precisely, if the parameter to the built-in eval function is a String, it is
treated as an ECMAScript Script. The
eval code for a particular invocation of eval is the global code portion of that
Script.
then the source
text matched by the BindingIdentifier (if any) of that
declaration or expression is also included in the function code of the corresponding
function.
Function code is generally provided as the bodies of Function Definitions (15.2), Arrow Function Definitions
(15.3), Method Definitions
(15.4), Generator Function Definitions
(15.5), Async Function
Definitions (15.8), Async Generator
Function Definitions (15.6),
and Async Arrow Functions (15.9).
Function code is also derived from the arguments to the Function constructor
(20.2.1.1), the GeneratorFunction
constructor (27.3.1.1), the AsyncFunction constructor
(27.7.1.1), and the
AsyncGeneratorFunction constructor (27.4.1.1).
Note 2
The practical effect of including the BindingIdentifier in function code is
that the Early Errors for strict mode code are applied to a
BindingIdentifier
that is the name of a function whose body contains a "use strict" directive, even if the
surrounding code is not strict mode code.
11.2.1 Directive Prologues and the Use Strict Directive
An ECMAScript syntactic unit may be processed using either unrestricted or strict mode syntax and
semantics (4.3.2). Code is interpreted as
strict mode code in the following situations:
An ECMAScript implementation may support the evaluation of function exotic objects
whose evaluative behaviour is expressed in some host-defined form of executable code other than
ECMAScript source
text. Whether a function object is defined within ECMAScript code
or is a built-in function is not observable from the perspective of ECMAScript code that calls
or is called by such a function object.
12 ECMAScript Language: Lexical Grammar
The source text of an ECMAScript Script or
Module is first converted into a sequence of
input elements, which are tokens, line terminators, comments, or white space. The source text is scanned
from left to right, repeatedly taking the longest possible sequence of code points as the next input
element.
The use of multiple lexical goals ensures that there are no lexical ambiguities that would affect
automatic semicolon insertion. For example, there are no syntactic grammar contexts where both a
leading division or division-assignment, and a leading RegularExpressionLiteral are
permitted. This is not affected by semicolon insertion (see 12.10); in examples such as the
following:
a = b
/hi/g.exec(c).map(d);
where the first non-whitespace, non-comment code point after a LineTerminator is U+002F (SOLIDUS) and the
syntactic context allows division or division-assignment, no semicolon is inserted at the
LineTerminator. That is, the
above example is interpreted in the same way as:
The Unicode format-control characters (i.e., the characters in category “Cf” in the Unicode Character
Database such as LEFT-TO-RIGHT MARK or RIGHT-TO-LEFT MARK) are control codes used to control the
formatting of a range of text in the absence of higher-level protocols for this (such as mark-up
languages).
It is useful to allow format-control characters in source text to facilitate editing and display. All
format control characters may be used within comments, and within string literals, template
literals, and regular expression literals.
U+FEFF (ZERO WIDTH NO-BREAK SPACE) is a format-control character used primarily at the start of a
text to mark it as Unicode and to allow detection of the text's encoding and byte order.
<ZWNBSP> characters intended for this purpose can sometimes also appear after the start of a
text, for example as a result of concatenating files. In ECMAScript source text <ZWNBSP>
code points are treated as white space characters (see 12.2) outside of comments, string
literals, template literals, and regular expression literals.
12.2 White Space
White space code points are used to improve source text readability and to separate tokens
(indivisible lexical units) from each other, but are otherwise insignificant. White space code
points may occur between any two tokens and at the start or end of input. White space code points
may occur within a StringLiteral,
a RegularExpressionLiteral, a Template, or a TemplateSubstitutionTail where they are
considered significant code points forming part of a literal value. They may also occur within a
Comment, but cannot appear within any
other kind of token.
The ECMAScript white space code points are listed in Table 35.
Table 35: White Space Code Points
Code Points
Name
Abbreviation
U+0009
CHARACTER TABULATION
<TAB>
U+000B
LINE TABULATION
<VT>
U+000C
FORM FEED (FF)
<FF>
U+FEFF
ZERO WIDTH NO-BREAK SPACE
<ZWNBSP>
any code point in general category “Space_Separator”
<USP>
Note 1
U+0020 (SPACE) and U+00A0 (NO-BREAK SPACE) code points are part of <USP>.
Note 2
Other than for the code points listed in Table 35,
ECMAScript WhiteSpace
intentionally excludes all code points that have the Unicode “White_Space” property but
which are not classified in general category “Space_Separator” (“Zs”).
Like white space code points, line terminator code points are used to improve source text readability
and to separate tokens (indivisible lexical units) from each other. However, unlike white space code
points, line terminators have some influence over the behaviour of the syntactic grammar. In
general, line terminators may occur between any two tokens, but there are a few places where they
are forbidden by the syntactic grammar. Line terminators also affect the process of automatic
semicolon insertion (12.10). A line terminator cannot
occur within any token except a StringLiteral, Template, or TemplateSubstitutionTail. <LF> and
<CR> line terminators cannot occur within a StringLiteral token except as part of a LineContinuation.
Line terminators are included in the set of white space code points that are matched by the
\s class in regular expressions.
The ECMAScript line terminator code points are listed in Table 36.
Table 36: Line Terminator Code Points
Code Point
Unicode Name
Abbreviation
U+000A
LINE FEED (LF)
<LF>
U+000D
CARRIAGE RETURN (CR)
<CR>
U+2028
LINE SEPARATOR
<LS>
U+2029
PARAGRAPH SEPARATOR
<PS>
Only the Unicode code points in Table 36 are treated as line
terminators. Other new line or line breaking Unicode code points are not treated as line terminators
but are treated as white space if they meet the requirements listed in Table 35. The sequence
<CR><LF> is commonly used as a line terminator. It should be considered a single SourceCharacter for the purpose of
reporting line numbers.
Comments can be either single or multi-line. Multi-line comments cannot nest.
Because a single-line comment can contain any Unicode code point except a LineTerminator code point, and because of the
general rule that a token is always as long as possible, a single-line comment always consists of
all code points from the // marker to the end of the line. However, the LineTerminator at the end of the
line is not considered to be part of the single-line comment; it is recognized separately by the
lexical grammar and becomes part of the stream of input elements for the syntactic grammar. This
point is very important, because it implies that the presence or absence of single-line comments
does not affect the process of automatic semicolon insertion (see 12.10).
Comments behave like white space and are discarded except that, if a MultiLineComment contains a line terminator code
point, then the entire comment is considered to be a LineTerminator for purposes of parsing by the
syntactic grammar.
IdentifierName and ReservedWord are tokens that are
interpreted according to the Default Identifier Syntax given in Unicode Standard Annex #31,
Identifier and Pattern Syntax, with some small modifications. ReservedWord is an enumerated subset of IdentifierName. The syntactic
grammar defines Identifier as an
IdentifierName that is not a
ReservedWord. The Unicode
identifier grammar is based on character properties specified by the Unicode Standard. The Unicode
code points in the specified categories in the latest version of the Unicode Standard must be
treated as in those categories by all conforming ECMAScript implementations. ECMAScript
implementations may recognize identifier code points defined in later editions of the Unicode
Standard.
Note 1
This standard specifies specific code point additions: U+0024 (DOLLAR SIGN) and U+005F (LOW
LINE) are permitted anywhere in an IdentifierName.
The sets of code points with Unicode properties “ID_Start” and “ID_Continue” include,
respectively, the code points with Unicode properties “Other_ID_Start” and
“Other_ID_Continue”.
12.7.1 Identifier Names
Unicode escape sequences are permitted in an IdentifierName, where they contribute a single
Unicode code point equal to the IdentifierCodePoint of the UnicodeEscapeSequence.
The \ preceding the UnicodeEscapeSequence does not
contribute any code points. A UnicodeEscapeSequence cannot be used to
contribute a code point to an IdentifierName that would otherwise be invalid.
In other words, if a \UnicodeEscapeSequence sequence were
replaced by the SourceCharacter it contributes, the result
must still be a valid IdentifierName that has the exact same sequence
of SourceCharacter elements
as the original IdentifierName. All interpretations of IdentifierName within this
specification are based upon their actual code points regardless of whether or not an escape
sequence was used to contribute any particular code point.
Two IdentifierNames that are
canonically equivalent according to the Unicode Standard are not equal unless, after
replacement of each UnicodeEscapeSequence, they are
represented by the exact same sequence of code points.
The syntax-directed
operation IdentifierCodePoints takes no arguments and returns a
List of code points.
It is defined piecewise over the following productions:
Return the code point whose numeric value is the MV of CodePoint.
12.7.2 Keywords and Reserved Words
A keyword is a token that matches IdentifierName, but also has a
syntactic use; that is, it appears literally, in a fixed width font, in some
syntactic production. The keywords of ECMAScript include if, while,
async, await, and many others.
A reserved word is an IdentifierName that cannot be
used as an identifier. Many keywords are reserved words, but some are not, and some are reserved
only in certain contexts. if and while are reserved words.
await is reserved only inside async functions and modules. async is
not reserved; it can be used as a variable name or statement label without restriction.
This specification uses a combination of grammatical productions and early error rules
to specify which names are valid identifiers and which are reserved words. All tokens in the
ReservedWord list below,
except for await and yield, are unconditionally reserved. Exceptions
for await and yield are specified in 13.1, using parameterized syntactic
productions. Lastly, several early error rules restrict the set of valid
identifiers. See 13.1.1, 14.3.1.1,
14.7.5.1,
and 15.7.1. In
summary, there are five categories of identifier names:
Those that are always allowed as identifiers, and are not keywords, such as
Math, window, toString, and _;
Those that are never allowed as identifiers, namely the ReservedWords listed below except
await and yield;
Those that are contextually allowed as identifiers, namely await and
yield;
Those that are contextually disallowed as identifiers, in strict
mode code: let, static,
implements, interface, package,
private, protected, and public;
Those that are always allowed as identifiers, but also appear as keywords within certain
syntactic productions, at places where Identifier is not allowed: as,
async, from, get, meta,
of, set, and target.
The term conditional keyword, or contextual keyword, is sometimes used to
refer to the keywords that fall in the last three categories, and thus can be used as
identifiers in some contexts and as keywords in others.
Per 5.1.5, keywords in the grammar match
literal sequences of specific SourceCharacter elements. A code point
in a keyword cannot be expressed by a \UnicodeEscapeSequence.
enum is not currently used as a keyword in this specification. It is a
future reserved word, set aside for use as a keyword in future language
extensions.
Similarly, implements, interface, package,
private, protected, and public are future
reserved words in strict mode code.
The syntax-directed
operation NumericValue takes no arguments and returns a Number or a
BigInt. It is defined piecewise over the following productions:
A string literal is 0 or more Unicode code points enclosed in single or double quotes.
Unicode code points may also be represented by an escape sequence. All code points may
appear literally in a string literal except for the closing quote code points, U+005C
(REVERSE SOLIDUS), U+000D (CARRIAGE RETURN), and U+000A (LINE FEED). Any code points may
appear in the form of an escape sequence. String literals evaluate to ECMAScript String
values. When generating these String values Unicode code points are UTF-16 encoded as
defined in 11.1.1. Code points belonging to
the Basic Multilingual Plane are encoded as a single code unit element of the string.
All other code points are encoded as two code unit elements of the string.
<LF> and <CR> cannot appear in a string literal, except as part of a LineContinuation to
produce the empty code points sequence. The proper way to include either in the String
value of a string literal is to use an escape sequence such as \n or
\u000A.
It is possible for string literals to precede a Use
Strict Directive that places the enclosing code in strict mode, and implementations
must take care to enforce the above rules for such literals. For example, the
following source text contains a Syntax Error:
A string literal stands for a value of the String type. SV
produces String values for string literals through recursive application on the various
parts of the string literal. As part of this process, some Unicode code points within the
string literal are interpreted as having a mathematical value, as
described below or in 12.9.3.
A regular expression literal is an input element that is converted to a RegExp object
(see 22.2) each time the
literal is evaluated. Two regular expression literals in a program evaluate to regular
expression objects that never compare as === to each other even if the two
literals' contents are identical. A RegExp object may also be created at runtime by
new RegExp or calling the RegExp constructor as a function (see
22.2.4).
The productions below describe the syntax for a regular expression literal and are used by the
input element scanner to find the end of the regular expression literal. The source text
comprising the RegularExpressionBody and the RegularExpressionFlags
are subsequently parsed again using the more stringent ECMAScript Regular Expression grammar
(22.2.1).
An implementation may extend the ECMAScript Regular Expression grammar defined in 22.2.1, but it
must not extend the RegularExpressionBody and RegularExpressionFlags
productions defined below or the productions used by these productions.
Regular expression literals may not be empty; instead of representing an empty regular
expression literal, the code unit sequence // starts a single-line comment.
To specify an empty regular expression, use: /(?:)/.
12.9.5.1 Static Semantics: BodyText
The syntax-directed
operation BodyText takes no arguments and returns source text. It is
defined piecewise over the following productions:
The syntax-directed
operation TV takes no arguments and returns a String or
undefined. A template literal component is interpreted by TV as a value
of the String type. TV is
used to construct the indexed components of a template object (colloquially, the template
values). In TV, escape sequences are replaced by the UTF-16 code unit(s) of the Unicode code
point represented by the escape sequence.
The syntax-directed
operation TRV takes no arguments and returns a String. A template
literal component is interpreted by TRV as a value of the String type. TRV is
used to construct the raw components of a template object (colloquially, the template raw
values). TRV is similar to TV with the difference being that in
TRV, escape sequences are interpreted as they appear in the literal.
The TRV of HexDigit::one of0123456789abcdefABCDEF is the result of performing UTF16EncodeCodePoint on the
single code point matched by this production.
Most ECMAScript statements and declarations must be terminated with a semicolon. Such semicolons may
always appear explicitly in the source text. For convenience, however, such semicolons may be
omitted from the source text in certain situations. These situations are described by saying that
semicolons are automatically inserted into the source code token stream in those situations.
12.10.1 Rules of Automatic Semicolon Insertion
In the following rules, “token” means the actual recognized lexical token determined using the
current lexical goal symbol as described in clause
12.
There are three basic rules of semicolon insertion:
When, as the source text is parsed from left to right, a token (called the offending
token) is encountered that is not allowed by any production of the grammar,
then a semicolon is automatically inserted before the offending token if one or more of
the following conditions is true:
The offending token is separated from the previous token by at least one LineTerminator.
The offending token is }.
The previous token is ) and the inserted semicolon would then be parsed
as the terminating semicolon of a do-while statement (14.7.2).
When, as the source text is parsed from left to right, the end of the input stream of tokens
is encountered and the parser is unable to parse the input token stream as a single instance
of the goal nonterminal, then a semicolon is automatically inserted at the end of the input
stream.
When, as the source text is parsed from left to right, a token is encountered that is
allowed by some production of the grammar, but the production is a restricted
production and the token would be the first token for a terminal or nonterminal
immediately following the annotation “[no LineTerminator here]” within the restricted
production (and therefore such a token is called a restricted token), and the restricted
token is separated from the previous token by at least one LineTerminator, then a semicolon is
automatically inserted before the restricted token.
However, there is an additional overriding condition on the preceding rules: a semicolon is never
inserted automatically if the semicolon would then be parsed as an empty statement or if that
semicolon would become one of the two semicolons in the header of a for statement
(see 14.7.4).
Note
The following are the only restricted productions in the grammar:
The practical effect of these restricted productions is as follows:
When a ++ or -- token is encountered where the parser
would treat it as a postfix operator, and at least one LineTerminator occurred between the
preceding token and the ++ or -- token, then a semicolon
is automatically inserted before the ++ or -- token.
When a continue, break, return,
throw, or yield token is encountered and a LineTerminator is
encountered before the next token, a semicolon is automatically inserted after the
continue, break, return, throw,
or yield token.
When arrow function parameter(s) are followed by a LineTerminator before a
=> token, a semicolon is automatically inserted and the punctuator
causes a syntax error.
When an async token is followed by a LineTerminator before a
function or IdentifierName or (
token, a semicolon is automatically inserted and the async token is not
treated as part of the same expression or class element as the following tokens.
When an async token is followed by a LineTerminator before a
* token, a semicolon is automatically inserted and the punctuator
causes a syntax error.
The resulting practical advice to ECMAScript programmers is:
A postfix ++ or -- operator should be on the same line as
its operand.
An Expression in a
return or throw statement or an AssignmentExpression in a
yield expression should start on the same line as the
return, throw, or yield token.
A LabelIdentifier in a
break or continue statement should be on the same line as
the break or continue token.
The end of an arrow function's parameter(s) and its => should be on
the same line.
The async token preceding an asynchronous function or method should be
on the same line as the immediately following token.
12.10.2 Examples of Automatic Semicolon Insertion
This section is non-normative.
The source
{ 12 } 3
is not a valid sentence in the ECMAScript grammar, even with the automatic semicolon insertion
rules. In contrast, the source
{ 12 } 3
is also not a valid ECMAScript sentence, but is transformed by automatic semicolon insertion into
the following:
{ 1
;2 ;} 3;
which is a valid ECMAScript sentence.
The source
for (a; b
)
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion because
the semicolon is needed for the header of a for statement. Automatic semicolon
insertion never inserts one of the two semicolons in the header of a for statement.
The source
return
a + b
is transformed by automatic semicolon insertion into the following:
return;
a + b;
Note 1
The expression a + b is not treated as a value to be returned by the
return statement, because a LineTerminator separates it from the
token return.
The source
a = b
++c
is transformed by automatic semicolon insertion into the following:
a = b;
++c;
Note 2
The token ++ is not treated as a postfix operator applying to the variable
b, because a LineTerminator occurs between
b and ++.
The source
if (a > b)
else c = d
is not a valid ECMAScript sentence and is not altered by automatic semicolon insertion before the
else token, even though no production of the grammar applies at that point, because
an automatically inserted semicolon would then be parsed as an empty statement.
The source
a = b + c
(d + e).print()
is not transformed by automatic semicolon insertion, because the parenthesized
expression that begins the second line can be interpreted as an argument list for a function
call:
a = b + c(d + e).print()
In the circumstance that an assignment statement must begin with a left parenthesis, it is a good
idea for the programmer to provide an explicit semicolon at the end of the preceding statement
rather than to rely on automatic semicolon insertion.
12.10.3 Interesting Cases of Automatic Semicolon Insertion
This section is non-normative.
ECMAScript programs can be written in a style with very few semicolons by relying on automatic
semicolon insertion. As described above, semicolons are not inserted at every newline, and
automatic semicolon insertion can depend on multiple tokens across line terminators.
As new syntactic features are added to ECMAScript, additional grammar productions could be added
that cause lines relying on automatic semicolon insertion preceding them to change grammar
productions when parsed.
For the purposes of this section, a case of automatic semicolon insertion is considered
interesting if it is a place where a semicolon may or may not be inserted, depending on the
source text which precedes it. The rest of this section describes a number of interesting cases
of automatic semicolon insertion in this version of ECMAScript.
12.10.3.1 Interesting Cases of Automatic Semicolon Insertion in
Statement Lists
In a StatementList, many
StatementListItems
end in semicolons, which may be omitted using automatic semicolon insertion. As a
consequence of the rules above, at the end of a line ending an expression, a semicolon is
required if the following line begins with any of the following:
An opening parenthesis ((). Without a semicolon, the two
lines together are treated as a CallExpression.
An opening square bracket ([). Without a semicolon, the
two lines together are treated as property access, rather than an ArrayLiteral or ArrayAssignmentPattern.
A template literal (`). Without a semicolon, the two lines
together are interpreted as a tagged Template (13.3.11), with the
previous expression as the MemberExpression.
Unary + or -. Without a semicolon, the two
lines together are interpreted as a usage of the corresponding binary operator.
A RegExp literal. Without a semicolon, the two lines together may be
parsed instead as the /MultiplicativeOperator, for
example if the RegExp has flags.
12.10.3.2 Cases of Automatic Semicolon Insertion and “[no
LineTerminator here]”
This section is non-normative.
ECMAScript contains grammar productions which include “[no LineTerminator here]”. These productions
are sometimes a means to have optional operands in the grammar. Introducing a LineTerminator in these
locations would change the grammar production of a source text by using the grammar
production without the optional operand.
The rest of this section describes a number of productions using “[no LineTerminator here]” in
this version of ECMAScript.
12.10.3.2.1 List of Grammar Productions with Optional
Operands and “[no LineTerminator here]”
yield and await are permitted as BindingIdentifier in the grammar, and
prohibited with static semantics below, to prohibit
automatic semicolon insertion in cases such as
It is a Syntax Error if IsStrict(this phrase) is true
and the StringValue of IdentifierName is one of
"implements", "interface", "let",
"package", "private", "protected",
"public", "static", or "yield".
An ArrayLiteral is an
expression describing the initialization of an Array, using a list, of zero or more
expressions each of which represents an array element, enclosed in square brackets. The
elements need not be literals; they are evaluated each time the array initializer is
evaluated.
Array elements may be elided at the beginning, middle or end of the element list. Whenever a
comma in the element list is not preceded by an AssignmentExpression (i.e., a comma at
the beginning or after another comma), the missing array element contributes to the length of
the Array and increases the index of subsequent elements. Elided array elements are not defined.
If an element is elided at the end of an array, that element does not contribute to the length
of the Array.
CreateDataPropertyOrThrow
is used to ensure that own properties are defined for the array even if the standard
built-in Array
prototype object has been modified in a manner that would
preclude the creation of new own properties using [[Set]].
An object initializer is an expression describing the initialization of an Object,
written in a form resembling a literal. It is a list of zero or more pairs of property
keys and associated values, enclosed in curly brackets. The
values need not be literals; they are evaluated each time the object initializer is
evaluated.
In certain contexts, ObjectLiteral is used as a cover grammar
for a more restricted secondary grammar. The CoverInitializedName production
is necessary to fully cover these secondary grammars. However, use of this production
results in an early Syntax Error in normal contexts where an actual ObjectLiteral is
expected.
It is a Syntax Error if any source text is matched by this production.
Note 1
This production exists so that ObjectLiteral can serve as a cover
grammar for ObjectAssignmentPattern.
It cannot occur in an actual object initializer.
The syntax-directed
operation PropertyNameList takes no arguments and returns a List of Strings. It is
defined piecewise over the following productions:
The abstract operation IsValidRegularExpressionLiteral takes argument literal (a
RegularExpressionLiteralParse Node) and returns a Boolean. It
determines if its argument is a valid regular expression literal. It performs the following
steps when called:
Set patternText to the sequence of code points resulting from
interpreting each of the 16-bit elements of stringValue as a
Unicode BMP code point. UTF-16 decoding is not applied to the elements.
Let parseResult be ParsePattern(patternText,
u, v).
If parseResult is a Parse Node,
return true; else return false.
The syntax-directed
operation TemplateStrings takes argument raw (a Boolean)
and returns a List of either Strings
or undefined. It is defined piecewise over the following productions:
This operation returns undefined if raw is
false and templateToken contains a NotEscapeSequence. In all other
cases, it returns a String.
13.2.8.4 GetTemplateObject ( templateLiteral )
The abstract operation GetTemplateObject takes argument templateLiteral (a
Parse Node) and returns an Array. It
performs the following steps when called:
Append the Record { [[Site]]: templateLiteral, [[Array]]: template } to
realm.[[TemplateMap]].
Return template.
Note 1
The creation of a template object cannot result in an abrupt
completion.
Note 2
Each TemplateLiteral in the program
code of a realm is associated with a unique template
object that is used in the evaluation of tagged Templates (13.2.8.6).
The template objects are frozen and the same template object is used each time a
specific tagged Template is evaluated. Whether template objects are created lazily
upon first evaluation of the TemplateLiteral or eagerly prior
to first evaluation is an implementation choice that is not observable to ECMAScript
code.
Note 3
Future editions of this specification may define additional non-enumerable properties
of template objects.
This algorithm does not apply GetValue to Evaluation of Expression. The
principal motivation for this is so that operators such as delete and
typeof may be applied to parenthesized expressions.
Let propertyNameReference be ? Evaluation of expression.
Let propertyNameValue be ? GetValue(propertyNameReference).
NOTE: In most cases, ToPropertyKey will be performed on
propertyNameValue immediately after this step. However, in the case of
a[b] = c, it will not be performed until after evaluation of
c.
Return the Reference Record
{ [[Base]]: baseValue, [[ReferencedName]]: propertyNameValue, [[Strict]]: strict, [[ThisValue]]: empty }.
The abstract operation EvaluatePropertyAccessWithIdentifierKey takes arguments
baseValue (an ECMAScript language value),
identifierName (an IdentifierNameParse
Node), and strict (a Boolean) and returns a Reference Record. It
performs the following steps when called:
Let propertyNameString be the StringValue of
identifierName.
Return the Reference Record
{ [[Base]]: baseValue, [[ReferencedName]]: propertyNameString, [[Strict]]: strict, [[ThisValue]]: empty }.
NOTE: In most cases, ToPropertyKey will be performed on
propertyNameValue immediately after this step. However, in the case of
super[b] = c, it will not be performed until after evaluation of
c.
The abstract operation EvaluateImportCall takes argument specifierExpression (a
Parse Node) and optional argument
optionsExpression (a Parse Node) and returns either a
normal completion
containing a Promise or an abrupt completion.
It performs the following steps when called:
Perform ! Call(promiseCapability.[[Reject]], undefined, «
a newly created TypeError object »).
Return promiseCapability.[[Promise]].
Sort attributes according to the lexicographic order of their
[[Key]] field, treating the value of each such
field as a sequence of UTF-16 code unit values. NOTE: This sorting is
observable only in that hosts are prohibited from changing
behaviour based on the order in which attributes are enumerated.
Let moduleRequest be a new ModuleRequest
Record { [[Specifier]]:
specifierString, [[Attributes]]:
attributes }.
The abstract operation ContinueDynamicImport takes arguments promiseCapability (a
PromiseCapability Record) and
moduleCompletion (either a normal completion
containing a Module Record or a
throw completion)
and returns unused. It completes the process of a dynamic import
originally started by an import() call, resolving or
rejecting the promise returned by that call as appropriate. It performs the following steps
when called:
Let linkAndEvaluateClosure be a new Abstract Closure with no
parameters that captures module, promiseCapability, and
onRejected and performs the following steps when called:
Let fulfilledClosure be a new Abstract Closure with no
parameters that captures module and promiseCapability
and performs the following steps when called:
A tagged template is a function call where the arguments of the call are derived from a
TemplateLiteral
(13.2.8). The actual arguments
include a template object (13.2.8.4) and the values produced
by evaluating the expressions embedded within the TemplateLiteral.
The host-defined abstract operation
HostFinalizeImportMeta takes arguments importMeta (an Object) and
moduleRecord (a Module Record)
and returns unused. It allows hosts to perform any extraordinary
operations to prepare the object returned from import.meta.
Most hosts will be
able to simply define HostGetImportMetaProperties,
and leave HostFinalizeImportMeta with its default behaviour. However,
HostFinalizeImportMeta provides an "escape hatch" for hosts which need to directly
manipulate the object before it is exposed to ECMAScript code.
The default implementation of HostFinalizeImportMeta is to return
unused.
When a delete operator occurs within strict mode code, a
SyntaxError exception is thrown if its UnaryExpression is a direct
reference to a variable, function argument, or function name. In addition, if a
delete operator occurs within strict mode code
and the property to be deleted has the attribute { [[Configurable]]: false } (or otherwise
cannot be deleted), a TypeError exception is thrown.
Note 2
The object that may be created in step 4.c is not accessible
outside of the above abstract operation and the ordinary object[[Delete]] internal method. An implementation might choose
to avoid the actual creation of that object.
The result of evaluating a relational operator is always of type Boolean, reflecting whether
the relationship named by the operator holds between its two operands.
The abstract operation InstanceofOperator takes arguments V (an ECMAScript language value) and
target (an ECMAScript language value) and
returns either a normal completion
containing a Boolean or a throw completion. It
implements the generic algorithm for determining if V is an instance of
target either by consulting target's %Symbol.hasInstance% method or, if absent,
determining whether the value of target's "prototype" property is
present in V's prototype chain. It performs the following steps when called:
Steps 4 and 5 provide compatibility with
previous editions of ECMAScript that did not use a %Symbol.hasInstance% method to
define the instanceof operator semantics. If an object does not define or
inherit %Symbol.hasInstance% it uses the
default instanceof semantics.
13.11 Equality Operators
Note
The result of evaluating an equality operator is always of type Boolean, reflecting whether
the relationship named by the operator holds between its two operands.
If r is true, return false. Otherwise,
return true.
Note 1
Given the above definition of equality:
String comparison can be forced by: `${a}` == `${b}`.
Numeric comparison can be forced by: +a == +b.
Boolean comparison can be forced by: !a == !b.
Note 2
The equality operators maintain the following invariants:
A != B is equivalent to !(A == B).
A == B is equivalent to B == A, except in the order of
evaluation of A and B.
Note 3
The equality operator is not always transitive. For example, there might be two distinct
String objects, each representing the same String value; each String object would be
considered equal to the String value by the == operator, but the two String
objects would not be equal to each other. For example:
new String("a") == "a" and "a" == new String("a") are both
true.
new String("a") == new String("a") is false.
Note 4
Comparison of Strings uses a simple equality test on sequences of code unit values. There
is no attempt to use the more complex, semantically oriented definitions of character or
string equality and collating order defined in the Unicode specification. Therefore
Strings values that are canonically equal according to the Unicode Standard could test
as unequal. In effect this algorithm assumes that both Strings are already in normalized
form.
The value produced by a && or || operator is not
necessarily of type Boolean. The value produced will always be the value of one of the two
operand expressions.
The grammar for a ConditionalExpression in ECMAScript
is slightly different from that in C and Java, which each allow the second subexpression to
be an Expression but
restrict the third expression to be a ConditionalExpression. The
motivation for this difference in ECMAScript is to allow an assignment expression to be
governed by either arm of a conditional and to eliminate the confusing and fairly useless
case of a comma expression as the centre expression.
When this expression occurs within strict mode code, it
is a runtime error if lRef in step 1.d,
2,
2,
2,
2
is an unresolvable reference. If it is, a ReferenceError exception is
thrown. Additionally, it is a runtime error if the lRef in step 8,
6,
6,
6
is a reference to a data property with the attribute value {
[[Writable]]: false }, to an accessor
property with the attribute value { [[Set]]: undefined }, or to a non-existent
property of an object for which the IsExtensible predicate returns the
value false. In these cases a TypeError exception
is thrown.
No hint is provided in the calls to ToPrimitive in steps 1.a and 1.b. All standard objects
except Dates handle the absence of a hint as if number were
given; Dates handle the absence of a hint as if string were
given. Exotic
objects may handle the absence of a hint in some other manner.
Note 2
Step 1.c differs from step
3 of the IsLessThan
algorithm, by using the logical-or operation instead of the logical-and operation.
The abstract operation EvaluateStringOrNumericBinaryExpression takes arguments
leftOperand (a Parse Node), opText (a sequence
of Unicode code points), and rightOperand (a Parse Node) and returns
either a normal completion
containing either a String, a BigInt, or a Number, or an abrupt completion. It
performs the following steps when called:
The value of a StatementList is the value of the last
value-producing item in the StatementList. For example, the
following calls to the eval function all return the value 1:
The abstract operation BlockDeclarationInstantiation takes arguments code (a Parse
Node) and env (a Declarative Environment
Record) and returns unused. code is the
Parse
Node corresponding to the body of the block. env is the
Environment Record in which bindings are
to be created.
When undefined is passed for environment it indicates
that a PutValue operation should be used to
assign the initialization value. This is the case for formal parameter lists of
non-strict functions. In that
case the formal parameter bindings are preinitialized in order to deal with the
possibility of multiple parameters with the same name.
It is defined piecewise over the following productions:
The lookahead-restriction [lookahead ≠ else] resolves the
classic "dangling else" problem in the usual way. That is, when the choice of associated
if is otherwise ambiguous, the else is associated with the nearest
(innermost) of the candidate ifs
The abstract operation LoopContinues takes arguments completion (a Completion Record)
and labelSet (a List of
Strings) and returns a Boolean. It performs the following steps when called:
The abstract operation CreatePerIterationEnvironment takes argument
perIterationBindings (a List of
Strings) and returns either a normal completion
containingunused or a throw completion. It
performs the following steps when called:
undefined is passed for environment to indicate that a
PutValue operation should be used to
assign the initialization value. This is the case for var statements
and the formal parameter lists of some non-strict
functions (see 10.2.11). In
those cases a lexical binding is hoisted and preinitialized prior to evaluation of
its initializer.
It is defined piecewise over the following productions:
The abstract operation ForIn/OfBodyEvaluation takes arguments lhs (a Parse
Node), stmt (a StatementParse
Node), iteratorRecord (an Iterator
Record), iterationKind (enumerate
or iterate), lhsKind (assignment,
var-binding, or lexical-binding), and
labelSet (a List of
Strings) and optional argument iteratorKind (sync or
async) and returns either a normal completion
containing an ECMAScript language
value or an abrupt completion.
It performs the following steps when called:
If iteratorKind is not present, set iteratorKind to
sync.
The abstract operation EnumerateObjectProperties takes argument O (an Object) and
returns an iterator object. It performs the
following steps when called:
Return an iterator object whose
next method iterates over all the String-valued keys of enumerable
properties of O. The iterator object
is never directly accessible to ECMAScript code. The mechanics and order of
enumerating the properties is not specified but must conform to the rules specified
below.
The iterator's throw and
return methods are null and are never invoked. The iterator's next method
processes object properties to determine whether the property key should be returned as
an iterator value. Returned property keys
do not include keys that are Symbols. Properties of the target object may be deleted during
enumeration. A property that is deleted before it is processed by the iterator's next method is
ignored. If new properties are added to the target object during enumeration, the newly
added properties are not guaranteed to be processed in the active enumeration. A property
name will be returned by the iterator's
next method at most once in any enumeration.
Enumerating the properties of the target object includes enumerating properties of its
prototype, and the prototype of the prototype, and so on, recursively; but a property of a
prototype is not processed if it has the same name as a property that has already been
processed by the iterator's next method.
The values of [[Enumerable]] attributes are not considered when
determining if a property of a prototype object has already been processed. The enumerable
property names of prototype objects must be obtained by invoking EnumerateObjectProperties
passing the prototype object as the argument. EnumerateObjectProperties must obtain the own
property
keys of the target object by calling its [[OwnPropertyKeys]] internal method. Property attributes of the
target object must be obtained by calling its [[GetOwnProperty]]
internal method.
the value of the [[Prototype]] internal slot of O or
an object in its prototype chain changes,
a property is removed from O or an object in its prototype chain,
a property is added to an object in O's prototype chain, or
the value of the [[Enumerable]] attribute of a property of
O or an object in its prototype chain changes.
Note 1
ECMAScript implementations are not required to implement the algorithm in 14.7.5.10.2.1
directly. They may choose any implementation whose behaviour will not deviate from
that algorithm unless one of the constraints in the previous paragraph is violated.
The following is an informative definition of an ECMAScript generator function that
conforms to these rules:
function* EnumerateObjectProperties(obj) {
const visited = newSet();
for (const key ofReflect.ownKeys(obj)) {
if (typeof key === "symbol") continue;
const desc = Reflect.getOwnPropertyDescriptor(obj, key);
if (desc) {
visited.add(key);
if (desc.enumerable) yield key;
}
}
const proto = Reflect.getPrototypeOf(obj);
if (proto === null) return;
for (const protoKey ofEnumerateObjectProperties(proto)) {
if (!visited.has(protoKey)) yield protoKey;
}
}
Note 2
The list of exotic objects for which implementations
are not required to match CreateForInIterator was chosen
because implementations historically differed in behaviour for those cases, and agreed
in all others.
14.7.5.10 For-In Iterator Objects
A For-In
Iterator is an object that represents a specific iteration over some specific
object. For-In Iterator objects are never directly accessible to ECMAScript code; they exist
solely to illustrate the behaviour of EnumerateObjectProperties.
14.7.5.10.1 CreateForInIterator ( object )
The abstract operation CreateForInIterator takes argument object (an Object)
and returns a For-In Iterator. It is used
to create a For-In Iterator object which
iterates over the own and inherited enumerable string properties of object in
a specific order. It performs the following steps when called:
It is a Syntax Error if this ContinueStatement is not nested,
directly or indirectly (but not crossing function or static initialization
block boundaries), within an IterationStatement.
It is a Syntax Error if this BreakStatement is not nested, directly or
indirectly (but not crossing function or static initialization block
boundaries), within an IterationStatement or a SwitchStatement.
A return statement causes a function to cease execution and, in most cases,
returns a value to the caller. If Expression is omitted, the return value is
undefined. Otherwise, the return value is the value of Expression. A
return statement may not actually return a value to the caller depending on
surrounding context. For example, in a try block, a return
statement's Completion Record
may be replaced with another Completion
Record during evaluation of the finally block.
The with statement adds an Object Environment
Record for a computed object to the lexical environment of the
running execution context. It then
executes a statement using this augmented lexical environment. Finally, it restores the
original lexical environment.
No matter how control leaves the embedded Statement, whether normally or by some form
of abrupt
completion or exception, the LexicalEnvironment is always
restored to its former state.
This operation does not execute C's StatementList (if any). The CaseBlock algorithm uses its
return value to determine which StatementList to start executing.
A Statement may be prefixed by
a label. Labelled statements are only used in conjunction with labelled break
and continue statements. ECMAScript has no goto statement. A
Statement can be part of a
LabelledStatement,
which itself can be part of a LabelledStatement, and so on. The labels
introduced this way are collectively referred to as the “current label set” when describing
the semantics of individual statements.
The abstract operation IsLabelledFunction takes argument stmt (a StatementParse
Node) and returns a Boolean. It performs the following steps when called:
The try statement encloses a block of code in which an exceptional condition can
occur, such as a runtime error or a throw statement. The catch
clause provides the exception-handling code. When a catch clause catches an exception, its
CatchParameter is bound
to that exception.
Evaluating a DebuggerStatement may allow an
implementation to cause a breakpoint when run under a debugger. If a debugger is not
present or active this statement has no observable effect.
Various ECMAScript language elements cause the creation of ECMAScript function
objects (10.2). Evaluation of such
functions starts with the execution of their [[Call]] internal method
(10.2.1).
The syntax-directed
operation ExpectedArgumentCount takes no arguments and returns a
non-negative integer.
It is defined piecewise over the following productions:
The ExpectedArgumentCount of a FormalParameterList is the number
of FormalParameters to the left of
either the rest parameter or the first FormalParameter with an Initializer. A
FormalParameter
without an initializer is allowed after the first parameter with an initializer but such
parameters are considered to be optional with undefined as their
default value.
The syntax-directed
operation FunctionBodyContainsUseStrict takes no arguments and returns a
Boolean. It is defined piecewise over the following productions:
A "prototype" property is automatically created for every function
defined using a FunctionDeclaration or FunctionExpression,
to allow for the possibility that the function will be used as a constructor.
The syntax-directed
operation ConciseBodyContainsUseStrict takes no arguments and returns a
Boolean. It is defined piecewise over the following productions:
An ArrowFunction does
not define local bindings for arguments, super,
this, or new.target. Any reference to arguments,
super, this, or new.target within an ArrowFunction must
resolve to a binding in a lexically enclosing environment. Typically this will be the
Function Environment of an immediately enclosing function. Even though an ArrowFunction may contain
references to super, the function object created in
step 5 is not
made into a method by performing MakeMethod. An ArrowFunction that references
super is always contained within a non-ArrowFunction and the necessary state to
implement super is accessible via the env that is captured by
the function
object of the ArrowFunction.
YieldExpression cannot
be used within the FormalParameters of a generator function
because any expressions that are part of FormalParameters are evaluated before the
resulting Generator is in a resumable state.
Let innerResult be ? Call(throw,
iterator, « received.[[Value]] »).
If generatorKind is async, set
innerResult to ? Await(innerResult).
NOTE: Exceptions from the inner iteratorthrow method are propagated. Normal
completions from an inner
throw method are processed similarly to an inner
next.
NOTE: If iterator does not have a throw
method, this throw is going to terminate the yield*
loop. But first we need to give iterator a chance to
clean up.
It is a Syntax Error if the PrivateBoundIdentifiers
of ClassElementList
contains any duplicate entries, unless the name is used once for a getter and once for a
setter and in no other entries, and the getter and setter are either both static or both
non-static.
The syntax-directed
operation ClassElementKind takes no arguments and returns
constructor-method, non-constructor-method, or
empty. It is defined piecewise over the following productions:
The syntax-directed
operation AllPrivateIdentifiersValid takes argument names (a
List of Strings) and
returns a Boolean.
Every grammar production alternative in this specification which is not listed below implicitly
has the following default definition of AllPrivateIdentifiersValid:
The syntax-directed
operation PrivateBoundIdentifiers takes no arguments and returns a
List of Strings. It is
defined piecewise over the following productions:
Every grammar production alternative in this specification which is not listed below implicitly
has the following default definition of ContainsArguments:
For ease of specification, private methods and accessors are included alongside private
fields in the [[PrivateElements]] slot of class instances.
However, any given object has either all or none of the private methods and accessors
defined by a given class. This feature has been designed so that implementations may
choose to implement private methods and accessors using a strategy which does not
require tracking each method or accessor individually.
For example, an implementation could directly associate instance private methods with
their corresponding Private Name and track, for each
object, which class constructors have run with that object as
their this value. Looking up an instance private method on an object then
consists of checking that the class constructor which defines the method has been
used to initialize the object, then returning the method associated with the Private
Name.
This differs from private fields: because field initializers can throw during class
instantiation, an individual object may have some proper subset of the private fields of
a given class, and so private fields must in general be tracked individually.
It is defined piecewise over the following productions:
NOTE: This branch behaves similarly to
constructor(...args) { super(...args); }. The
most notable distinction is that while the aforementioned
ECMAScript source
text observably calls the %Symbol.iterator%
method on %Array.prototype%, this function does
not.
Let func be ! F.[[GetPrototypeOf]]().
If IsConstructor(func)
is false, throw a
TypeError exception.
await is parsed as a keyword of an
AwaitExpression when
the [Await] parameter is present. The [Await] parameter is present in
the top level of the following contexts, although the parameter may be absent in some
contexts depending on the nonterminals, such as FunctionBody:
When Script is the syntactic
goal symbol, await may
be parsed as an identifier when the [Await] parameter is absent. This includes
the following contexts:
The syntax-directed
operation AsyncConciseBodyContainsUseStrict takes no arguments and
returns a Boolean. It is defined piecewise over the following productions:
Tail Position calls are only defined in strict mode code
because of a common non-standard language extension (see 10.2.4) that enables
observation of the chain of caller contexts.
call is a Parse Node that represents a
specific range of source text. When the following algorithms compare call to
another Parse Node, it is a test of whether
they represent the same source text.
Note 2
A potential tail position call that is immediately followed by return GetValue of
the call result is also a possible tail position call. A function call cannot return a
Reference Record,
so such a GetValue operation will always return the
same value as the actual function call result.
It is defined piecewise over the following productions:
The abstract operation PrepareForTailCall takes no arguments and returns
unused. It performs the following steps when called:
Assert: The
current execution context will not
subsequently be used for the evaluation of any ECMAScript code or built-in functions.
The invocation of Call subsequent to the invocation of this abstract operation will
create and push a new execution context before
performing any such evaluation.
Discard all resources associated with the current execution context.
Return unused.
A tail position call must either release any transient internal resources associated with the
currently executing function execution context before invoking the
target function or reuse those resources in support of the target function.
Note
For example, a tail position call should only grow an implementation's activation record
stack by the amount that the size of the target function's activation record exceeds the
size of the calling function's activation record. If the target function's activation
record is smaller, then the total size of the stack should decrease.
A map from the specifier strings imported by this script to the resolved
Module Record.
The list does not contain two different Recordsr1 and r2 such that ModuleRequestsEqual(r1,
r2) is true.
[[HostDefined]]
anything (default value is empty)
Field reserved for use by host
environments that need to associate additional
information with a script.
The abstract operation ParseScript takes arguments sourceText (ECMAScript source
text), realm (a Realm Record), and
hostDefined (anything) and returns a Script Record or a non-empty
List of
SyntaxError objects. It creates a Script Record based upon the result
of parsing sourceText as a Script. It performs the following steps when called:
An implementation may parse script source text and analyse it for Early Error conditions
prior to evaluation of ParseScript for that script source text. However, the reporting
of any errors must be deferred until the point where this specification actually
performs ParseScript upon that source text.
When an execution context is established
for evaluating scripts, declarations are instantiated in the current global environment.
Each global binding declared in the code is instantiated.
NOTE: Global var and function bindings (except those
that are introduced by non-strict direct
eval) are non-configurable and are therefore restricted
global properties.
If hasRestrictedGlobal is true, throw a
SyntaxError exception.
If vnDefinable is false,
throw a TypeError exception.
If declaredVarNames does not contain
vn, then
Append vn to
declaredVarNames.
NOTE: No abnormal terminations occur after this algorithm step if the global
object is an ordinary object. However, if the
global
object is a Proxy exotic object it
may exhibit behaviours that cause abnormal terminations in some of the following steps.
NOTE: Annex
B.3.2.2
adds additional steps at this point.
Early
errors specified in 16.1.1 prevent
name conflicts between function/var declarations and let/const/class declarations as
well as redeclaration of let/const/class bindings for declaration contained within a
single Script. However, such
conflicts and redeclarations that span more than one Script are detected as runtime errors during
GlobalDeclarationInstantiation. If any such errors are detected, no bindings are
instantiated for the script. However, if the global object is defined
using Proxy exotic objects then the runtime
tests for conflicting declarations may be unreliable resulting in an abrupt
completion and some global declarations not being instantiated.
If this occurs, the code for the Script is not evaluated.
Unlike explicit var or function declarations, properties that are directly created on the
global
object result in global bindings that may be shadowed by
let/const/class declarations.
The duplicate ExportedNames rule
implies that multiple export defaultExportDeclaration items within a
ModuleBody is a
Syntax Error. Additional error conditions relating to conflicting or duplicate
declarations are checked during module linking prior to evaluation of a Module. If any such errors
are detected the Module
is not evaluated.
The abstract operation ImportedLocalNames takes argument importEntries (a
List of ImportEntry
Records) and returns a List of Strings. It
creates a List of all of the
local name bindings defined by importEntries. It performs the following steps
when called:
A LoadedModuleRequest Record represents the request to import a module
together with the resulting Module Record. It consists of the
same fields defined in table Table 40, with the addition of
[[Module]]:
A Module Record encapsulates structural
information about the imports and exports of a single module. This information is used to
link the imports and exports of sets of connected modules. A Module Record includes four
fields that are only used when evaluating a module.
For specification purposes Module Record values are values of the Record specification
type and can be thought of as existing in a simple object-oriented hierarchy where Module
Record is an abstract class with both abstract and concrete subclasses. This specification
defines the abstract subclass named Cyclic Module Record and its concrete
subclass named Source Text Module Record. Other
specifications and implementations may define additional Module Record subclasses
corresponding to alternative module definition facilities that they defined.
Module Record defines the fields listed in Table 43. All Module
Definition subclasses include at least those fields. Module Record also defines the abstract
method list in Table 44. All Module
definition subclasses must provide concrete implementations of these abstract methods.
Prepares the module for linking by recursively loading all its
dependencies, and returns a promise.
GetExportedNames([exportStarSet])
Return a list of all names that are either directly or indirectly
exported from this module.
LoadRequestedModules must have completed successfully prior to
invoking this method.
ResolveExport(exportName [, resolveSet])
Return the binding of a name exported by this module. Bindings are
represented by a ResolvedBinding
Record, of the form { [[Module]]:
Module
Record, [[BindingName]]: String |
namespace }. If the export is a Module
Namespace Object without a direct binding in any module, [[BindingName]] will be set to
namespace. Return null if
the name cannot be resolved, or ambiguous if
multiple bindings were found.
Each time this operation is called with a specific
exportName, resolveSet pair as arguments it
must return the same result.
LoadRequestedModules must have completed successfully prior to
invoking this method.
Link()
Prepare the module for evaluation by transitively resolving all
module dependencies and creating a Module Environment
Record.
LoadRequestedModules must have completed successfully prior to
invoking this method.
Evaluate()
Returns a promise for the evaluation of this module and its
dependencies, resolving on successful evaluation or if it has
already been evaluated successfully, and rejecting for an evaluation
error or if it has already been evaluated unsuccessfully. If the
promise is rejected, hosts are expected to handle the
promise rejection and rethrow the evaluation error.
Link must have completed successfully prior to invoking this method.
16.2.1.5.1 EvaluateModuleSync ( module )
The abstract operation EvaluateModuleSync takes argument module (a Module Record) and returns
either a normal completion
containingunused or a throw
completion. It synchronously evaluates module,
provided that the caller guarantees that module's evaluation will return an
already settled promise. It performs the following steps when called:
A Cyclic Module
Record is used to represent information about a module that can participate in
dependency cycles with other modules that are subclasses of the Cyclic
Module Record type. Module Records that
are not subclasses of the Cyclic Module Record type must not
participate in dependency cycles with Source Text Module
Records.
new, unlinked,
linking, linked,
evaluating,
evaluating-async, or
evaluated
Initially new. Transitions to
unlinked, linking,
linked, evaluating,
possibly evaluating-async,
evaluated (in that order) as the module
progresses throughout its lifecycle.
evaluating-async indicates this module is queued
to execute on completion of its asynchronous dependencies or it is a
module whose [[HasTLA]] field is
true that has been executed and is pending top-level
completion.
Auxiliary field used during Link and Evaluate only. If [[Status]] is either
linking or evaluating,
this is either the module's depth-first traversal index or that of an
"earlier" module in the same strongly connected component.
A map from the specifier strings used by the module represented by this
record to request the importation of a module with the relative import
attributes to the resolved Module
Record. The list does not contain two different
Recordsr1 and r2 such that ModuleRequestsEqual(r1,
r2) is true.
The first visited module of the cycle, the root DFS ancestor of the
strongly connected component. For a module not in a cycle, this would be
the module itself. Once Evaluate has completed, a module's [[DFSAncestorIndex]] is the depth-first
traversal index of its [[CycleRoot]].
[[HasTLA]]
a Boolean
Whether this module is individually asynchronous (for example, if it's a
Source Text Module
Record containing a top-level await). Having an
asynchronous dependency does not mean this field is
true. This field must not change after the module is
parsed.
This field is initially set to unset, and remains
unset for fully synchronous modules. For modules
that are either themselves asynchronous or have an asynchronous
dependency, it is set to an integer that determines the order
in which execution of pending modules is queued by 16.2.1.6.1.3.4.
Once the pending module is executed, the field is set to
done.
If this module is the [[CycleRoot]] of some
cycle, and Evaluate() was called on some module in that cycle, this
field contains the PromiseCapability
Record for that entire evaluation. It is used to
settle the Promise object that is returned from the Evaluate() abstract
method. This field will be empty for any
dependencies of that module, unless a top-level Evaluate() has been
initiated for some of those dependencies.
If this module or a dependency has [[HasTLA]]true, and execution is in progress, this tracks the
parent importers of this module for the top-level execution job. These
parent modules will not start executing before this module has
successfully completed execution.
If this module has any asynchronous dependencies, this tracks the number
of asynchronous dependency modules remaining to execute for this module.
A module with asynchronous dependencies will be executed when this field
reaches 0 and there are no execution errors.
Evaluate the module's code within its execution context.
If this module has true in [[HasTLA]], then a PromiseCapability
Record is passed as an argument, and the method
is expected to resolve or reject the given capability. In this case, the
method must not throw an exception, but instead reject the PromiseCapability
Record if necessary.
A GraphLoadingState Record is a Record that contains
information about the loading process of a module graph. It's used to continue loading after
a call to HostLoadImportedModule. Each
GraphLoadingState Record has the
fields defined in Table 47:
It is a list of the Cyclic Module
Records that have been already loaded by the
current loading process, to avoid infinite loops with circular
dependencies.
The LoadRequestedModules concrete method of a Cyclic Module
Recordmodule takes optional argument
hostDefined (anything) and returns a Promise. It populates the [[LoadedModules]] of all the Module Records in the
dependency graph of module (most of the work is done by the auxiliary
function InnerModuleLoading). It takes
an optional hostDefined parameter that is passed to the HostLoadImportedModule
hook. It performs the following steps when called:
If hostDefined is not present, let hostDefined be
empty.
Let state be the GraphLoadingState
Record { [[IsLoading]]:
true, [[PendingModulesCount]]:
1, [[Visited]]: « », [[PromiseCapability]]: pc, [[HostDefined]]: hostDefined }.
The hostDefined parameter can be used to pass additional information
necessary to fetch the imported modules. It is used, for example, by HTML to set
the correct fetch destination for
<link rel="preload" as="..."> tags.
import() expressions never set the hostDefined
parameter.
The abstract operation InnerModuleLoading takes arguments state (a
GraphLoadingState
Record) and module (a Module Record) and
returns unused. It is used by LoadRequestedModules to
recursively perform the actual loading process for module's
dependency graph. It performs the following steps when called:
The Link concrete method of a Cyclic Module
Recordmodule takes no arguments and returns
either a normal
completion containingunused or a
throw
completion. On success, Link transitions this module's [[Status]] from unlinked to
linked. On failure, an exception is thrown and this module's
[[Status]] remains unlinked. (Most
of the work is done by the auxiliary function InnerModuleLinking.) It
performs the following steps when called:
Assert: module.[[Status]] is one of
unlinked, linked,
evaluating-async, or
evaluated.
16.2.1.6.1.2.1 InnerModuleLinking (
module, stack, index )
The abstract operation InnerModuleLinking takes arguments module (a
Module Record),
stack (a List of
Cyclic Module Records), and
index (a non-negative integer) and returns either a normal completion
containing a non-negative integer or a throw
completion. It is used by Link to perform the actual
linking process for module, as well as recursively on all other
modules in the dependency graph. The stack and index
parameters, as well as a module's [[DFSAncestorIndex]]
field, keep track of the depth-first search (DFS) traversal. In particular, [[DFSAncestorIndex]] is used to discover strongly
connected components (SCCs), such that all modules in an SCC transition to
linked together. It performs the following steps when
called:
If requiredModule and module are
the same Module
Record, set done to
true.
Return index.
16.2.1.6.1.3 Evaluate ( )
The Evaluate concrete method of a Cyclic Module
Recordmodule takes no arguments and returns a
Promise. Evaluate transitions this module's [[Status]] from
linked to either evaluating-async or
evaluated. The first time it is called on a module in a given
strongly connected component, Evaluate creates and returns a Promise which resolves
when the module has finished evaluating. This Promise is stored in the [[TopLevelCapability]] field of the [[CycleRoot]] for the component. Future invocations of
Evaluate on any module in the component return the same Promise. (Most of the work
is done by the auxiliary function InnerModuleEvaluation.) It
performs the following steps when called:
Assert: This call to Evaluate is not
happening at the same time as another call to Evaluate within the surrounding agent.
Assert: module.[[Status]] is one of linked,
evaluating-async, or
evaluated.
If module.[[Status]] is either
evaluating-async or evaluated,
set module to module.[[CycleRoot]].
If module.[[TopLevelCapability]] is not
empty, then
16.2.1.6.1.3.1 InnerModuleEvaluation (
module, stack, index )
The abstract operation InnerModuleEvaluation takes arguments module (a
Module Record),
stack (a List of
Cyclic Module Records), and
index (a non-negative integer) and returns either a normal completion
containing a non-negative integer or a throw
completion. It is used by Evaluate to perform the actual
evaluation process for module, as well as recursively on all other
modules in the dependency graph. The stack and index
parameters, as well as module's [[DFSAncestorIndex]] field, are used the same way as in
InnerModuleLinking. It
performs the following steps when called:
Assert:
requiredModule.[[AsyncEvaluationOrder]] is
either an integer or
unset.
If requiredModule.[[AsyncEvaluationOrder]] is
unset, set
requiredModule.[[Status]] to
evaluated.
Otherwise, set requiredModule.[[Status]] to
evaluating-async.
If requiredModule and module are
the same Module
Record, set done to
true.
Set requiredModule.[[CycleRoot]] to
module.
Return index.
Note 1
A module is evaluating while it is being traversed
by InnerModuleEvaluation. A module is evaluated
on execution completion or evaluating-async
during execution if its [[HasTLA]] field is
true or if it has asynchronous dependencies.
Note 2
Any modules depending on a module of an asynchronous cycle when that
cycle is not evaluating will instead depend on
the execution of the root of the cycle via [[CycleRoot]]. This ensures that the cycle state
can be treated as a single strongly connected component through its root
module state.
16.2.1.6.1.3.2 ExecuteAsyncModule (
module )
The abstract operation ExecuteAsyncModule takes argument module (a
Cyclic Module Record) and
returns unused. It performs the following steps when
called:
Assert: module.[[Status]] is either
evaluating or
evaluating-async.
The abstract operation GatherAvailableAncestors takes arguments module
(a Cyclic Module Record) and
execList (a List of
Cyclic Module Records) and
returns unused. It performs the following steps when
called:
When an asynchronous execution for a root module is fulfilled,
this function determines the list of modules which are able to
synchronously execute together on this completion, populating them in
execList.
The abstract operation AsyncModuleExecutionFulfilled takes argument
module (a Cyclic Module Record) and
returns unused. It performs the following steps when
called:
Assert: All elements of
execList have their [[AsyncEvaluationOrder]] field set to an
integer, [[PendingAsyncDependencies]] field set to 0, and
[[EvaluationError]] field set to
empty.
Let sortedExecList be a List
whose elements are the elements of execList, sorted by their
[[AsyncEvaluationOrder]] field in ascending
order.
The abstract operation AsyncModuleExecutionRejected takes arguments
module (a Cyclic Module Record) and
error (an ECMAScript language
value) and returns unused. It
performs the following steps when called:
NOTE: module.[[AsyncEvaluationOrder]] is set to
done for symmetry with AsyncModuleExecutionFulfilled.
In InnerModuleEvaluation,
the value of a module's [[AsyncEvaluationOrder]] internal slot is unused
when its [[EvaluationError]] internal slot is
not empty.
This non-normative section gives a series of examples of the linking and evaluation of a
few common module graphs, with a specific focus on how errors can occur.
First consider the following simple module graph:
Figure 2: A simple module graph
Let's first assume that there are no error conditions. When a host first calls
A.LoadRequestedModules(), this will complete successfully by assumption, and
recursively load the dependencies of B and C as well
(respectively, C and none), and then set A.[[Status]] = B.[[Status]] =
C.[[Status]] = unlinked.
Then, when the host
calls A.Link(), it will complete successfully (again by assumption) such that
A.[[Status]] = B.[[Status]] = C.[[Status]] =
linked. These preparatory steps can be performed at any time. Later, when the host is ready to incur
any possible side effects of the modules, it can call A.Evaluate(), which
will complete successfully, returning a Promise resolving to
undefined (again by assumption), recursively having evaluated first
C and then B. Each module's [[Status]] at
this point will be evaluated.
Consider then cases involving linking errors, after a successful call to
A.LoadRequestedModules(). If InnerModuleLinking
of C succeeds but, thereafter, fails for B, for example because it
imports something that C does not provide, then the original
A.Link() will fail, and both A and B's [[Status]] remain unlinked.
C's [[Status]] has become
linked, though.
Finally, consider a case involving evaluation errors after a successful call to Link().
If InnerModuleEvaluation of
C succeeds but, thereafter, fails for B, for example because
B contains code that throws an exception, then the original
A.Evaluate() will fail, returning a rejected Promise. The resulting exception
will be recorded in both A and B's [[EvaluationError]] fields, and their [[Status]] will become evaluated.
C will also become evaluated but, in contrast to
A and B, will remain without an [[EvaluationError]], as it successfully completed evaluation.
Storing the exception ensures that any time a host tries to reuse A or B by
calling their Evaluate() method, it will encounter the same exception. (Hosts are not required to
reuse Cyclic Module Records; similarly,
hosts are not
required to expose the exception objects thrown by these methods. However, the
specification enables such uses.)
Now consider a different type of error condition:
Figure 3: A module graph with an unresolvable module
In this scenario, module A declares a dependency on some other module, but no
Module Record exists for that
module, i.e. HostLoadImportedModule calls
FinishLoadingImportedModule
with an exception when asked for it. This could occur for a variety of reasons, such as
the corresponding resource not existing, or the resource existing but ParseModule returning some errors when
trying to parse the resulting source text. Hosts can choose to expose the cause of failure via
the completion they pass to FinishLoadingImportedModule.
In any case, this exception causes a loading failure, which results in A's
[[Status]] remaining new.
The difference here between loading, linking and evaluation errors is due to the
following characteristic:
Evaluation must be only performed
once, as it can cause side effects; it is thus important to remember whether
evaluation has already been performed, even if unsuccessfully. (In the error case,
it makes sense to also remember the exception because otherwise subsequent
Evaluate() calls would have to synthesize a new one.)
Linking, on the other hand, is side-effect-free, and thus even if it fails, it can
be retried at a later time with no issues.
Loading closely interacts with the host, and it may be desirable for some of them
to allow users to retry failed loads (for example, if the failure is caused by
temporarily bad network conditions).
Now, consider a module graph with a cycle:
Figure 4: A cyclic module graph
Here we assume that the entry point is module A, so that the host proceeds by calling
A.LoadRequestedModules(), which performs InnerModuleLoading
on A. This in turn calls InnerModuleLoading
on B and C. Because of the cycle, this again triggers InnerModuleLoading on
A, but at this point it is a no-op since A's dependencies loading
has already been triggered during this LoadRequestedModules process. When all the
modules in the graph have been successfully loaded, their [[Status]] transitions from new to
unlinked at the same time.
Then the host
proceeds by calling A.Link(), which performs InnerModuleLinking on
A. This in turn calls InnerModuleLinking on
B. Because of the cycle, this again triggers InnerModuleLinking on
A, but at this point it is a no-op since A.[[Status]] is already linking.
B.[[Status]] itself remains
linking when control gets back to A and InnerModuleLinking is triggered on
C. After this returns with C.[[Status]]
being linked, both A and B transition from
linking to linked together; this is by
design, since they form a strongly connected component. It's possible to transition the
status of modules in the same SCC at the same time because during this phase the module
graph is traversed with a depth-first search.
An analogous story occurs for the evaluation phase of a cyclic module graph, in the
success case.
Now consider a case where A has a linking error; for example, it tries to
import a binding from C that does not exist. In that case, the above steps
still occur, including the early return from the second call to InnerModuleLinking on
A. However, once we unwind back to the original InnerModuleLinking on
A, it fails during InitializeEnvironment, namely right after
C.ResolveExport(). The thrown SyntaxError exception
propagates up to A.Link, which resets all modules that are currently on its
stack (these are always exactly the modules that are still
linking). Hence both A and B become
unlinked. Note that C is left as
linked.
Alternatively, consider a case where A has an evaluation error; for example,
its source code throws an exception. In that case, the evaluation-time analogue of the
above steps still occurs, including the early return from the second call to InnerModuleEvaluation on
A. However, once we unwind back to the original InnerModuleEvaluation on
A, it fails by assumption. The exception thrown propagates up to
A.Evaluate(), which records the error in all modules that are currently on
its stack (i.e., the modules that are still
evaluating) as well as via [[AsyncParentModules]], which form a chain for modules which
contain or depend on top-level await through the whole dependency graph
through the AsyncModuleExecutionRejected
algorithm. Hence both A and B become
evaluated and the exception is recorded in both A and
B's [[EvaluationError]] fields, while C
is left as evaluated with no [[EvaluationError]].
Lastly, consider a module graph with a cycle, where all modules complete asynchronously:
Figure 5: An asynchronous cyclic module graph
Loading and linking happen as before, and all modules end up with [[Status]] set to linked.
Calling A.Evaluate() calls InnerModuleEvaluation on
A, B, and D, which all transition to
evaluating. Then InnerModuleEvaluation is called
on A again, which is a no-op because it is already
evaluating. At this point, D.[[PendingAsyncDependencies]] is 0, so ExecuteAsyncModule(D)
is called and we call D.ExecuteModule with a new PromiseCapability tracking
the asynchronous execution of D. We unwind back to the InnerModuleEvaluation on
B, setting B.[[PendingAsyncDependencies]]
to 1 and B.[[AsyncEvaluationOrder]] to 1. We unwind
back to the original InnerModuleEvaluation on
A, setting A.[[PendingAsyncDependencies]]
to 1. In the next iteration of the loop over A's dependencies, we call
InnerModuleEvaluation on
C and thus on D (again a no-op) and E. As E
has no dependencies and is not part of a cycle, we call ExecuteAsyncModule(E)
in the same manner as D and E is immediately removed from the
stack. We unwind once more to the InnerModuleEvaluation on
C, setting C.[[AsyncEvaluationOrder]] to
3. Now we finish the loop over A's dependencies, set A.[[AsyncEvaluationOrder]] to 4, and remove the entire strongly
connected component from the stack, transitioning all of the modules to
evaluating-async at once. At this point, the fields of the
modules are as given in Table
48.
Table 48: Module fields after the initial Evaluate() call
Field
Module
A
B
C
D
E
[[DFSAncestorIndex]]
0
0
0
0
4
[[Status]]
evaluating-async
evaluating-async
evaluating-async
evaluating-async
evaluating-async
[[AsyncEvaluationOrder]]
4
1
3
0
2
[[AsyncParentModules]]
« »
« A »
« A »
« B, C »
« C »
[[PendingAsyncDependencies]]
2 (B and C)
1 (D)
2 (D and E)
0
0
Let us assume that E finishes executing first. When that happens, AsyncModuleExecutionFulfilled
is called, E.[[Status]] is set to
evaluated and C.[[PendingAsyncDependencies]] is decremented to become 1. The
fields of the updated modules are as given in Table 49.
Table 49: Module fields after module E finishes executing
Field
Module
C
E
[[DFSAncestorIndex]]
0
4
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
3
done
[[AsyncParentModules]]
« A »
« C »
[[PendingAsyncDependencies]]
1 (D)
0
D is next to finish (as it was the only module that was still executing). When
that happens, AsyncModuleExecutionFulfilled
is called again and D.[[Status]] is set to
evaluated. Its ancestors available for execution are B
(whose [[AsyncEvaluationOrder]] is 1) and C (whose
[[AsyncEvaluationOrder]] is 3), thus B will be
handled first: B.[[PendingAsyncDependencies]] is
decremented to become 0, ExecuteAsyncModule is called on
B, and it starts executing. C.[[PendingAsyncDependencies]] is also decremented to become 0,
and C starts executing (potentially in parallel to B if
B contains an await). The fields of the updated modules are as
given in Table 50.
Table 50: Module fields after module D finishes executing
Field
Module
B
C
D
[[DFSAncestorIndex]]
0
0
0
[[Status]]
evaluating-async
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
1
3
done
[[AsyncParentModules]]
« A »
« A »
« B, C »
[[PendingAsyncDependencies]]
0
0
0
Let us assume that C finishes executing next. When that happens, AsyncModuleExecutionFulfilled
is called again, C.[[Status]] is set to
evaluated and A.[[PendingAsyncDependencies]] is decremented to become 1. The
fields of the updated modules are as given in Table 51.
Table 51: Module fields after module C finishes executing
Field
Module
A
C
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
4
done
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
1 (B)
0
Then, B finishes executing. When that happens, AsyncModuleExecutionFulfilled
is called again and B.[[Status]] is set to
evaluated. A.[[PendingAsyncDependencies]] is decremented to become 0, so
ExecuteAsyncModule is called and
it starts executing. The fields of the updated modules are as given in Table 52.
Table 52: Module fields after module B finishes executing
Field
Module
A
B
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluating-async
evaluated
[[AsyncEvaluationOrder]]
4
done
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
0
0
Finally, A finishes executing. When that happens, AsyncModuleExecutionFulfilled
is called again and A.[[Status]] is set to
evaluated. At this point, the Promise in A.[[TopLevelCapability]] (which was returned from
A.Evaluate()) is resolved, and this concludes the handling of this module
graph. The fields of the updated module are as given in Table 53.
Table 53: Module fields after module A finishes executing
Field
Module
A
[[DFSAncestorIndex]]
0
[[Status]]
evaluated
[[AsyncEvaluationOrder]]
done
[[AsyncParentModules]]
« »
[[PendingAsyncDependencies]]
0
Alternatively, consider a failure case where C fails execution and returns an
error before B has finished executing. When that happens, AsyncModuleExecutionRejected
is called, which sets C.[[Status]] to
evaluated and C.[[EvaluationError]] to the error. It then propagates this error
to all of the AsyncParentModules by performing AsyncModuleExecutionRejected
on each of them. The fields of the updated modules are as given in Table 54.
Table 54: Module fields after module C finishes with an error
Field
Module
A
C
[[DFSAncestorIndex]]
0
0
[[Status]]
evaluated
evaluated
[[AsyncEvaluationOrder]]
done
done
[[AsyncParentModules]]
« »
« A »
[[PendingAsyncDependencies]]
1 (B)
0
[[EvaluationError]]
empty
C's evaluation error
A will be rejected with the same error as C since C will
call AsyncModuleExecutionRejected
on A with C's error. A.[[Status]] is set to evaluated. At this
point the Promise in A.[[TopLevelCapability]] (which
was returned from A.Evaluate()) is rejected. The fields of the updated module
are as given in Table
55.
Table 55: Module fields after module A is rejected
Then, B finishes executing without an error. When that happens, AsyncModuleExecutionFulfilled
is called again and B.[[Status]] is set to
evaluated. GatherAvailableAncestors
is called on B. However, A.[[CycleRoot]]
is A which has an evaluation error, so it will not be added to the returned
sortedExecList and AsyncModuleExecutionFulfilled
will return without further processing. Any future importer of B will resolve
the rejection of B.[[CycleRoot]].[[EvaluationError]] from the evaluation error from C
that was set on the cycle root A. The fields of the updated modules are as
given in Table 56.
Table 56: Module fields after module B finishes executing in
an erroring graph
A Source
Text Module Record is used to represent information about a module that was
defined from ECMAScript source text (11) that was parsed using
the goal symbolModule. Its fields contain digested
information about the names that are imported and exported by the module, and its concrete
methods use these digests to link and evaluate the module.
A List
of ExportEntry records derived from the code of this module that
correspond to reexported imports that occur within the module or exports
from export * as namespace declarations.
A List
of ExportEntry records derived from the code of this module that
correspond to export * declarations that occur within the
module, not including export * as namespace declarations.
An ImportEntry
Record is a Record
that digests information about a single declarative import. Each ImportEntry
Record has the fields defined in Table 58:
The name under which the desired binding is exported by the module
identified by [[ModuleRequest]]. The value
namespace-object indicates that the import
request is for the target module's namespace object.
[[LocalName]]
a String
The name that is used to locally access the imported value from within
the importing module.
Note 1
Table
59 gives examples of ImportEntry records fields used to
represent the syntactic import forms:
An ExportEntry
Record is a Record
that digests information about a single declarative export. Each ExportEntry
Record has the fields defined in Table 60:
The name under which the desired binding is exported by the module
identified by [[ModuleRequest]].
null if the ExportDeclaration
does not have a ModuleSpecifier.
all is used for
export * as ns from "mod" declarations.
all-but-default is used for
export * from "mod" declarations.
[[LocalName]]
a String or null
The name that is used to locally access the exported value from within
the importing module. null if the exported value is
not locally accessible from within the module.
Note 2
Table
61 gives examples of the ExportEntry record fields used to
represent the syntactic export forms:
An implementation may parse module source text and analyse it for Early Error
conditions prior to the evaluation of ParseModule for that module source text.
However, the reporting of any errors must be deferred until the point where this
specification actually performs ParseModule upon that source text.
16.2.1.7.2 Implementation of Module Record Abstract Methods
The ResolveExport concrete method of a Source Text Module
Recordmodule takes argument exportName
(a String) and optional argument resolveSet (a List of
Records with
fields [[Module]] (a Module Record) and [[ExportName]] (a String)) and returns a ResolvedBinding Record,
null, or ambiguous.
ResolveExport attempts to resolve an imported binding to the actual defining module
and local binding name. The defining module may be the module represented by the
Module Record this method
was invoked on or some other module that is imported by that module. The parameter
resolveSet is used to detect unresolved circular import/export paths. If
a pair consisting of specific Module
Record and exportName is reached that is already
in resolveSet, an import circularity has been encountered. Before
recursively calling ResolveExport, a pair consisting of module and
exportName is added to resolveSet.
If a defining module is found, a ResolvedBinding
Record { [[Module]], [[BindingName]] } is returned. This record identifies the
resolved binding of the originally requested export, unless this is the export of a
namespace with no local binding. In this case, [[BindingName]] will be set to
namespace. If no definition was found or the request is found
to be circular, null is returned. If the request is found to be
ambiguous, ambiguous is returned.
Assert: There
is more than one * import that includes
the requested name.
If resolution.[[Module]] and
starResolution.[[Module]] are not the same
Module
Record, return
ambiguous.
If resolution.[[BindingName]] is not
starResolution.[[BindingName]] and either
resolution.[[BindingName]] or
starResolution.[[BindingName]] is
namespace, return
ambiguous.
If resolution.[[BindingName]]is
a String,
starResolution.[[BindingName]]is
a String, and
resolution.[[BindingName]] is not
starResolution.[[BindingName]], return
ambiguous.
Return starResolution.
16.2.1.7.3 Implementation of Cyclic Module Record Abstract
Methods
A Synthetic Module Record is
used to represent information about a module that is defined by specifications. Its exported
names are statically defined at creation, while their corresponding values can change over
time using SetSyntheticModuleExport. It has
no imports or dependencies.
Note
A Synthetic Module Record could be used for defining a variety of
module types: for example, JSON modules or CSS modules.
In addition to the fields defined in Table 43 Synthetic
Module Records have the additional fields listed in Table 62.
The initialization logic to perform upon evaluation of the module,
taking the Synthetic
Module Record as its sole argument. It must not
modify [[ExportNames]]. It may return an
abrupt
completion.
The abstract operation SetSyntheticModuleExport takes arguments module (a
Synthetic Module Record),
exportName (a String), and exportValue (an ECMAScript language value)
and returns unused. It can be used to set or change the exported
value for an existing export of a Synthetic Module
Record. It performs the following steps when called:
The LoadRequestedModules concrete method of a Synthetic Module Recordmodule takes no arguments and returns a Promise. It performs the
following steps when called:
The GetExportedNames concrete method of a Synthetic Module Recordmodule takes no arguments and returns a List of
Strings. It performs the following steps when called:
Return module.[[ExportNames]].
16.2.1.8.4.3 ResolveExport ( exportName )
The ResolveExport concrete method of a Synthetic Module
Recordmodule takes argument exportName
(a String) and returns a ResolvedBinding Record or
null. It performs the following steps when called:
If module.[[ExportNames]] does not
contain exportName, return null.
The actual process performed is host-defined, but typically consists of
performing whatever I/O operations are necessary to load the appropriate Module Record. Multiple different
(referrer, moduleRequest.[[Specifier]],
moduleRequest.[[Attributes]]) triples may map to the
same Module Record instance. The
actual mapping semantics is host-defined but typically a normalization
process is applied to specifier as part of the mapping process. A typical
normalization process would include actions such as expansion of relative and abbreviated
path specifiers.
Note 2
The above text requires that hosts support JSON modules when imported with
type: "json" (and HostLoadImportedModule completes normally), but it
does not prohibit hosts from supporting JSON modules when imported
without type: "json".
16.2.1.11 FinishLoadingImportedModule ( referrer,
moduleRequest, payload, result )
Append the LoadedModuleRequest
Record { [[Specifier]]: moduleRequest.[[Specifier]], [[Attributes]]:
moduleRequest.[[Attributes]],
[[Module]]: result.[[Value]] } to referrer.[[LoadedModules]].
The abstract operation AllImportAttributesSupported takes argument attributes (a
List of ImportAttribute Records) and returns a
Boolean. It performs the following steps when called:
If supported does not contain attribute.[[Key]], return false.
Return true.
16.2.1.12.1 HostGetSupportedImportAttributes ( )
The host-defined abstract operation
HostGetSupportedImportAttributes takes no arguments and returns a List of Strings.
It allows host environments to specify which
import attributes they support. Only attributes with supported keys will be provided to
the host.
An implementation of HostGetSupportedImportAttributes must conform to the following
requirements:
It must return a List of
Strings, each indicating a supported attribute.
Each time this operation is called, it must return the same List with the
same contents in the same order.
The default implementation of HostGetSupportedImportAttributes is to return a new empty
List.
Note
The purpose of requiring the host to specify its supported
import attributes, rather than passing all attributes to the host and letting it then choose
which ones it wants to handle, is to ensure that unsupported attributes are handled
in a consistent way across different hosts.
16.2.1.13 GetModuleNamespace ( module )
The abstract operation GetModuleNamespace takes argument module (an instance of a
concrete subclass of Module Record) and returns a
Module Namespace Object. It retrieves the Module Namespace Object representing
module's exports, lazily creating it the first time it was requested, and storing
it in module.[[Namespace]] for future retrieval. It
performs the following steps when called:
GetModuleNamespace never throws. Instead, unresolvable names are simply excluded from
the namespace at this point. They will lead to a real linking error later unless
they are all ambiguous star exports that are not explicitly requested anywhere.
Sort attributes according to the lexicographic order of their [[Key]] field, treating the value of each such field as a
sequence of UTF-16 code unit values. NOTE: This sorting is observable only in that
hosts are
prohibited from changing behaviour based on the order in which attributes are
enumerated.
Return a List whose
sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.
Return a List whose
sole element is a new ExportEntry Record { [[ModuleRequest]]: null, [[ImportName]]: null, [[LocalName]]: localName, [[ExportName]]: "default" }.
Return a List whose
sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: sourceName }.
Return a List whose
sole element is a new ExportEntry Record { [[ModuleRequest]]: module, [[ImportName]]: importName, [[LocalName]]: localName, [[ExportName]]: exportName }.
An implementation must report most errors at the time the relevant ECMAScript language construct is
evaluated. An early error is an error
that can be detected and reported prior to the evaluation of any construct in the Script containing the error. The presence of an
early error prevents
the evaluation of the construct. An implementation must report early errors in a Script as part of parsing that Script in ParseScript. Early errors in a Module are reported at the point when the Module would be evaluated and the Module is never initialized. Early errors in eval
code are reported at the time eval is called and prevent evaluation of the eval
code. All errors that are not early
errors are runtime errors.
An implementation must report as an early error any occurrence of a condition that is listed in a
“Static Semantics: Early Errors” subclause of this specification.
An implementation shall not treat other kinds of errors as early errors even if the compiler can prove
that a construct cannot execute without error under any circumstances. An implementation may issue an
early warning in such a case, but it should not report the error until the relevant construct is
actually executed.
An implementation shall report all errors as specified, except for the following:
Except as restricted in 17.1, a host or implementation may extend Script syntax, Module syntax, and regular expression pattern or flag
syntax. To permit this, all operations (such as calling eval, using a regular
expression literal, or using the Function or RegExp constructor) that are allowed to throw
SyntaxError are permitted to exhibit host-defined behaviour instead of throwing
SyntaxError when they encounter a host-defined extension to the script
syntax or regular expression pattern or flag syntax.
Except as restricted in 17.1, a host or implementation may provide additional
types, values, objects, properties, and functions beyond those described in this specification. This
may cause constructs (such as looking up a variable in the global scope) to have host-defined
behaviour instead of throwing an error (such as ReferenceError).
17.1 Forbidden Extensions
An implementation must not extend this specification in the following ways:
If an implementation extends any function object with an own property named
"caller" the value of that property, as observed using [[Get]] or [[GetOwnProperty]], must not be a
strict
function object. If it is an accessor property, the function
that is the value of the property's [[Get]] attribute must never return
a strict
function when called.
Neither mapped nor unmapped arguments objects may be created with an own property named
"caller".
The behaviour of built-in methods which are specified in ECMA-402, such as those named
toLocaleString, must not be extended except as specified in ECMA-402.
The RegExp pattern grammars in 22.2.1 and B.1.2 must not be extended to
recognize any of the source characters A-Z or a-z as IdentityEscape[+UnicodeMode]
when the [UnicodeMode] grammar parameter is present.
The Syntactic Grammar must not be extended in any manner that allows the token : to
immediately follow source text that is matched by the BindingIdentifier nonterminal symbol.
There are certain built-in objects available whenever an ECMAScript Script or Module begins execution. One, the global object,
is part of the global environment of the executing program. Others are accessible as initial properties
of the global
object or indirectly as properties of accessible built-in objects.
Unless specified otherwise, a built-in object that is callable as a function is a built-in function object
with the characteristics described in 10.3. Unless specified otherwise, the [[Extensible]] internal slot of a built-in object initially has the value
true. Every built-in function object has a [[Realm]]
internal slot whose value is the Realm Record of the realm for which the object was initially created.
Many built-in objects are functions: they can be invoked with arguments. Some of them furthermore are
constructors: they
are functions intended for use with the new operator. For each built-in function, this
specification describes the arguments required by that function and the properties of that function object.
For each built-in constructor, this specification furthermore describes
properties of the prototype object of that constructor and properties of specific object instances
returned by a new expression that invokes that constructor.
Unless otherwise specified in the description of a particular function, if a built-in function or
constructor is given
fewer arguments than the function is specified to require, the function or constructor shall behave exactly as if it had
been given sufficient additional arguments, each such argument being the undefined
value. Such missing arguments are considered to be “not present” and may be identified in that manner by
specification algorithms. In the description of a particular function, the terms
“this value” and “NewTarget” have the meanings given in 10.3.
Unless otherwise specified in the description of a particular function, if a built-in function or
constructor described
is given more arguments than the function is specified to allow, the extra arguments are evaluated by
the call and then ignored by the function. However, an implementation may define implementation specific
behaviour relating to such arguments as long as the behaviour is not the throwing of a
TypeError exception that is predicated simply on the presence of an extra argument.
Note 1
Implementations that add additional capabilities to the set of built-in functions are encouraged
to do so by adding new functions rather than adding new parameters to existing functions.
Unless otherwise specified every built-in function and every built-in constructor has the Function prototype
object, which is the initial value of the expression
Function.prototype (20.2.3), as
the value of its [[Prototype]] internal slot.
Unless otherwise specified every built-in prototype object has the Object prototype object,
which is the initial value of the expression Object.prototype (20.1.3), as the value of
its [[Prototype]] internal slot, except the Object prototype object
itself.
If this specification defines a built-in constructor's behaviour via algorithm steps, then that is its
behaviour for the purposes of both [[Call]] and [[Construct]]. If such an algorithm needs to distinguish the two cases, it
checks whether NewTarget is undefined, which indicates a [[Call]] invocation.
Built-in function
objects that are not identified as constructors do not implement the [[Construct]] internal method unless otherwise specified in the description of a
particular function.
Built-in function
objects that are not constructors do not have a "prototype"
property unless otherwise specified in the description of a particular function.
Each built-in function defined in this specification is created by calling the CreateBuiltinFunction abstract operation
(10.3.4). The values of the length
and name parameters are the initial values of the "length" and
"name" properties as discussed below. The values of the prefix parameter
are similarly discussed below.
Every built-in function
object, including constructors, has a "length" property
whose value is a non-negative integral Number. Unless otherwise specified, this value
is the number of required parameters shown in the subclause heading for the function description.
Optional parameters and rest parameters are not included in the parameter count.
Note 2
For example, the function object that is the initial value of the
"map" property of the Array prototype
object is described under the subclause heading «Array.prototype.map
(callback [ , thisArg])» which shows the two named arguments callback and thisArg, the latter
being optional; therefore the value of the "length" property of that
function
object is 1𝔽.
Unless otherwise specified, the "length" property of a built-in function object has
the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every built-in function
object, including constructors, has a "name" property whose
value is a String. Unless otherwise
specified, this value is the name that is given to the function in this specification. Functions that
are identified as anonymous functions use the empty String as the value of the "name"
property. For functions that are specified as properties of objects, the name value is the property name string
used to access the function. Functions that are specified as get or set accessor functions of built-in
properties have "get" or "set" (respectively) passed to the
prefix parameter when calling CreateBuiltinFunction.
The value of the "name" property is explicitly specified for each built-in functions
whose property keyis a Symbol value. If such an
explicitly specified value starts with the prefix "get " or "set "
and the function for which it is specified is a get or set accessor function of a built-in property, the
value without the prefix is passed to the name parameter, and the value
"get" or "set" (respectively) is passed to the prefix
parameter when calling CreateBuiltinFunction.
Unless otherwise specified, the "name" property of a built-in function object has
the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Every other data
property described in clauses 19 through 28 and in Annex
B.2 has the attributes { [[Writable]]: true, [[Enumerable]]:
false, [[Configurable]]: true } unless
otherwise specified.
Every accessor
property described in clauses 19 through 28 and in Annex
B.2 has the attributes { [[Enumerable]]: false, [[Configurable]]: true } unless otherwise specified. If only
a get accessor function is described, the set accessor function is the default value,
undefined. If only a set accessor is described the get accessor is the default value,
undefined.
does not have a [[Construct]] internal method; it cannot be used as a
constructor with
the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
has a [[Prototype]] internal slot whose value is host-defined.
may have host-defined properties in addition to the properties
defined in this specification. This may include a property whose value is the global object itself.
19.1 Value Properties of the Global Object
19.1.1 globalThis
The initial value of the "globalThis" property of the global
object in a Realm Recordrealm is
realm.[[GlobalEnv]].[[GlobalThisValue]].
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false, [[Configurable]]: true }.
19.1.2 Infinity
The value of Infinity is +∞𝔽 (see 6.1.6.1). This property
has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.1.3 NaN
The value of NaN is NaN (see 6.1.6.1). This property
has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
19.1.4 undefined
The value of undefined is undefined (see 6.1.1). This
property has the attributes { [[Writable]]: false,
[[Enumerable]]: false, [[Configurable]]: false }.
NOTE: If direct is true, runningContext
will be the execution context that
performed the direct
eval. If direct is false,
runningContext will be the execution
context for the invocation of the eval function.
The eval code cannot instantiate variable or function bindings in the variable
environment of the calling context that invoked the eval if either the code of the
calling context or the eval code is strict mode code.
Instead such bindings are instantiated in a new VariableEnvironment that is only
accessible to the eval code. Bindings introduced by let,
const, or class declarations are always instantiated in a
new LexicalEnvironment.
19.2.1.2 HostEnsureCanCompileStrings ( calleeRealm,
parameterStrings, bodyString, direct )
The host-defined abstract operation
HostEnsureCanCompileStrings takes arguments calleeRealm (a Realm
Record), parameterStrings (a List of Strings),
bodyString (a String), and direct (a Boolean) and returns either a
normal completion
containingunused or a throw completion. It
allows host
environments to block certain ECMAScript functions which allow
developers to interpret and evaluate strings as ECMAScript code.
parameterStrings represents the strings that, when using one of the function
constructors, will be concatenated together to
build the parameters list. bodyString represents the function body or the string
passed to an eval call.
direct signifies whether the evaluation is a direct eval.
The default implementation of HostEnsureCanCompileStrings is to return NormalCompletion(unused).
Let trimmedPrefix be the longest prefix of trimmed that satisfies
the syntax of a StrDecimalLiteral, which might be
trimmed itself. If there is no such prefix, return NaN.
This function may interpret only a leading portion of string as a Number
value; it ignores any code units that cannot be interpreted as part of the notation of a
decimal literal, and no indication is given that any such code units were ignored.
19.2.5 parseInt ( string, radix )
This function produces an integral Number dictated by interpretation of the
contents of string according to the specified radix. Leading white space
in string is ignored. If radix coerces to 0 (such as when it is
undefined), it is assumed to be 10 except when the number representation
begins with "0x" or "0X", in which case it is assumed to
be 16. If radix is 16, the number representation may optionally begin with
"0x" or "0X".
If S is not empty and the first code unit of S is the code unit
0x002D (HYPHEN-MINUS), set sign to -1.
If S is not empty and the first code unit of S is either the code
unit 0x002B (PLUS SIGN) or the code unit 0x002D (HYPHEN-MINUS), set S to the
substring
of S from index 1.
If S contains a code unit that is not a radix-R digit, let
end be the index within S of the first such code unit; otherwise,
let end be the length of S.
Let mathInt be the integer value that is represented by Z
in radix-R notation, using the letters A through Z and a
through z for digits with values 10 through 35. (However, if R = 10
and Z contains more than 20 significant digits, every significant digit after
the 20th may be replaced by a 0 digit, at the option of the implementation; and if
R is not one of 2, 4, 8, 10, 16, or 32, then mathInt may be an
implementation-approximatedinteger
representing the integer value denoted by Z in
radix-R notation.)
This function may interpret only a leading portion of string as an integer value; it
ignores any code units that cannot be interpreted as part of the notation of an
integer, and
no indication is given that any such code units were ignored.
19.2.6 URI Handling Functions
Uniform Resource Identifiers, or URIs, are Strings that identify resources (e.g. web pages or
files) and transport protocols by which to access them (e.g. HTTP or FTP) on the Internet. The
ECMAScript language itself does not provide any support for using URIs except for functions that
encode and decode URIs as described in this section. encodeURI and
decodeURI are intended to work with complete URIs; they assume that any reserved
characters are intended to have special meaning (e.g., as delimiters) and so are not encoded.
encodeURIComponent and decodeURIComponent are intended to work with
the individual components of a URI; they assume that any reserved characters represent text and
must be encoded to avoid special meaning when the component is part of a complete URI.
Note 1
The set of reserved characters is based upon RFC 2396 and does not reflect changes
introduced by the more recent RFC 3986.
Note 2
Many implementations of ECMAScript provide additional functions and methods that
manipulate web pages; these functions are beyond the scope of this standard.
19.2.6.1 decodeURI ( encodedURI )
This function computes a new version of a URI in which each escape sequence and UTF-8
encoding of the sort that might be introduced by the encodeURI function is
replaced with the UTF-16 encoding of the code point that it represents. Escape sequences
that could not have been introduced by encodeURI are not replaced.
This function computes a new version of a URI in which each escape sequence and UTF-8
encoding of the sort that might be introduced by the encodeURIComponent
function is replaced with the UTF-16 encoding of the code point that it represents.
It is the %decodeURIComponent% intrinsic object.
It performs the following steps when called:
Let componentString be ? ToString(encodedURIComponent).
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which
each instance of certain code points is replaced by one, two, three, or four escape
sequences representing the UTF-8 encoding of the code point.
This function computes a new version of a UTF-16 encoded (6.1.4) URI in which
each instance of certain code points is replaced by one, two, three, or four escape
sequences representing the UTF-8 encoding of the code point.
The abstract operation Encode takes arguments string (a String) and
extraUnescaped (a String) and returns either a normal completion
containing a String or a throw completion. It
performs URI encoding and escaping, interpreting string as a sequence of UTF-16
encoded code points as described in 6.1.4. If a
character is identified as unreserved in RFC 2396 or appears in extraUnescaped,
it is not escaped. It performs the following steps when called:
Because percent-encoding is used to represent individual octets, a single code point
may be expressed as multiple consecutive escape sequences (one for each of its 8-bit
UTF-8 code units).
19.2.6.6 Decode ( string,
preserveEscapeSet )
The abstract operation Decode takes arguments string (a String) and
preserveEscapeSet (a String) and returns either a normal completion
containing a String or a throw completion. It
performs URI unescaping and decoding, preserving any escape sequences that correspond to
Basic Latin characters in preserveEscapeSet. It performs the following steps when
called:
Let len be the length of string.
Let R be the empty String.
Let k be 0.
Repeat, while k < len,
Let C be the code unit at index k within
string.
Let S be C.
If C is the code unit 0x0025 (PERCENT SIGN), then
If k + 3 > len, throw a
URIError exception.
Let escape be the substring of
string from k to k + 3.
RFC 3629 prohibits the decoding of invalid UTF-8 octet sequences. For example, the
invalid sequence 0xC0 0x80 must not decode into the code unit 0x0000.
Implementations of the Decode algorithm are required to throw a
URIError when encountering such invalid sequences.
19.2.6.7 ParseHexOctet ( string, position
)
The abstract operation ParseHexOctet takes arguments string (a String) and
position (a non-negative integer) and returns either a non-negative integer or a non-empty
List of
SyntaxError objects. It parses a sequence of two hexadecimal characters
at the specified position in string into an unsigned 8-bit integer. It performs
the following steps when called:
callback should be a function that accepts two arguments.
groupBy calls callback once for each element in
items, in ascending order, and constructs a new object. Each value
returned by callback is coerced to a property key. For each
such property
key, the result object has a property whose key is that
property
key and whose value is an array containing all the elements
for which the callback return value coerced to that key.
callback is called with two arguments: the value of the element and the
index of the element.
The return value of groupBy is an object that does not inherit from
%Object.prototype%.
This function performs the following steps when called:
Let groups be ? GroupBy(items,
callback, property).
The ordering of steps 1 and
2 is chosen to ensure
that any exception that would have been thrown by step 1 in previous
editions of this specification will continue to be thrown even if the
this value is undefined or
null.
20.1.3.3 Object.prototype.isPrototypeOf ( V )
This method performs the following steps when called:
The ordering of steps 1 and
2 preserves the behaviour
specified by previous editions of this specification for the case where V
is not an object and the this value is
undefined or null.
20.1.3.4 Object.prototype.propertyIsEnumerable ( V )
This method performs the following steps when called:
This method does not consider objects in the prototype chain.
Note 2
The ordering of steps 1 and
2 is chosen to
ensure that any exception that would have been thrown by step 1 in previous
editions of this specification will continue to be thrown even if the
this value is undefined or
null.
The optional parameters to this method are not used but are intended to correspond to the
parameter pattern used by ECMA-402 toLocaleString methods. Implementations that
do not include ECMA-402 support must not use those parameter positions for other purposes.
Note 1
This method provides a generic toLocaleString implementation for objects
that have no locale-sensitive toString behaviour. Array,
Number, Date, and %TypedArray%
provide their own locale-sensitive toLocaleString methods.
Note 2
ECMA-402 intentionally does not provide an alternative to this default
implementation.
20.1.3.6 Object.prototype.toString ( )
This method performs the following steps when called:
If the this value is undefined, return
"[object Undefined]".
If the this value is null, return
"[object Null]".
Historically, this method was occasionally used to access the String value of the
[[Class]] internal slot that was used in previous editions
of this specification as a nominal type tag for various built-in objects. The above
definition of toString preserves compatibility for legacy code that
uses toString as a test for those specific kinds of built-in objects.
It does not provide a reliable type testing mechanism for other kinds of built-in or
program defined objects. In addition, programs can use %Symbol.toStringTag% in ways
that will invalidate the reliability of such legacy type tests.
20.1.3.7 Object.prototype.valueOf ( )
This method performs the following steps when called:
Object.prototype.__proto__ is an accessor property with
attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined
as follows:
20.1.3.8.1 get Object.prototype.__proto__
The value of the [[Get]] attribute is a built-in function that
requires no arguments. It performs the following steps when called:
is the initial value of the "Function" property of the global
object.
creates and initializes a new function object when called as a function
rather than as a constructor. Thus the function call
Function(…) is equivalent to the object creation expression
new Function(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Function behaviour must include a super call to the Function constructor to
create and initialize a subclass instance with the internal slots necessary for built-in
function behaviour. All ECMAScript syntactic forms for defining function
objects create instances of Function. There is no syntactic means to
create instances of Function subclasses except for the built-in GeneratorFunction,
AsyncFunction, and AsyncGeneratorFunction subclasses.
20.2.1.1 Function ( ...parameterArgs,
bodyArg )
The last argument (if any) specifies the body (executable code) of a function; any preceding
arguments specify formal parameters.
This function performs the following steps when called:
It is permissible but not necessary to have one argument for each formal parameter to
be specified. For example, all three of the following expressions produce the same
result:
The abstract operation CreateDynamicFunction takes arguments constructor (a
constructor), newTarget (a
constructor or undefined),
kind (normal, generator,
async, or async-generator),
parameterArgs (a List
of ECMAScript language
values), and bodyArg (an ECMAScript language value)
and returns either a normal completion
containing an ECMAScript function object or a
throw
completion. constructor is the constructor
function that is performing this action. newTarget is the constructor
that new was initially applied to. parameterArgs and
bodyArg reflect the argument values that were passed to
constructor. It performs the following steps when called:
If newTarget is undefined, set
newTarget to constructor.
If body is a List of
errors, throw a SyntaxError exception.
NOTE: The parameters and body are parsed separately to ensure that each is valid
alone. For example, new Function("/*", "*/ ) {") does not evaluate
to a function.
NOTE: If this step is reached, sourceText must have the syntax of
exprSym (although the reverse implication does not hold). The purpose
of the next two steps is to enforce any Early Error rules which apply to
exprSym directly.
NOTE: Functions whose kind is async are not
constructable and do not have a [[Construct]] internal
method or a "prototype" property.
Return F.
Note
CreateDynamicFunction defines a "prototype" property on any
function it creates whose kind is not async to
provide for the possibility that the function will be used as a constructor.
has a "name" property whose value is the empty String.
Note
The Function prototype object is specified to be a function object to ensure
compatibility with ECMAScript code that was created prior to the ECMAScript 2015
specification.
The thisArg value is passed without modification as the
this value. This is a change from Edition 3, where an
undefined or nullthisArg is
replaced with the global object and ToObject
is applied to all other values and that result is passed as the
this value. Even though the thisArg is passed without
modification, non-strict functions still
perform these transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function exotic
object, then the thisArg will be ignored by the
function [[Call]] in step 6.
Function
objects created using Function.prototype.bind
are exotic
objects. They also do not have a
"prototype" property.
Note 2
If Target is either an arrow function or a bound function exotic
object, then the thisArg passed to this method
will not be used by subsequent calls to F.
The thisArg value is passed without modification as the
this value. This is a change from Edition 3, where an
undefined or nullthisArg is
replaced with the global object and ToObject
is applied to all other values and that result is passed as the
this value. Even though the thisArg is passed without
modification, non-strict functions still
perform these transformations upon entry to the function.
Note 2
If func is either an arrow function or a bound function exotic
object, then the thisArg will be ignored by the
function [[Call]] in step 4.
20.2.3.4 Function.prototype.constructor
The initial value of Function.prototype.constructor is %Function%.
20.2.3.5 Function.prototype.toString ( )
This method performs the following steps when called:
Let func be the this value.
If funcis an Object, func has a
[[SourceText]] internal slot, func.[[SourceText]] is a sequence of Unicode code points, and
HostHasSourceTextAvailable(func)
is true, then
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
This is the default implementation of %Symbol.hasInstance% that most
functions inherit. %Symbol.hasInstance% is called by the
instanceof operator to determine whether a value is an instance of a
specific constructor. An expression such as
v instanceof F
evaluates as
F[%Symbol.hasInstance%](v)
A constructor function can control which
objects are recognized as its instances by instanceof by exposing a
different %Symbol.hasInstance% method on the function.
This property is non-writable and non-configurable to prevent tampering that could be used to
globally expose the target function of a bound function.
The value of the "name" property of this method is
"[Symbol.hasInstance]".
20.2.4 Function Instances
Every Function instance is an ECMAScript function object and has the internal slots listed
in Table 30.
Function
objects created using the Function.prototype.bind method
(20.2.3.2) have the internal slots
listed in Table
31.
Function instances have the following properties:
20.2.4.1 length
The value of the "length" property is an integral Number that indicates
the typical number of arguments expected by the function. However, the language permits the
function to be invoked with some other number of arguments. The behaviour of a function when
invoked on a number of arguments other than the number specified by its
"length" property depends on the function. This property has the
attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
20.2.4.2 name
The value of the "name" property is a String that is
descriptive of the function. The name has no semantic significance but is typically a
variable or property
name that is used to refer to the function at its point of definition
in ECMAScript
source text. This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: true }.
Anonymous functions objects that do not have a contextual name associated with them by this
specification use the empty String as the value of the "name" property.
20.2.4.3 prototype
Function instances that can be used as a constructor have a "prototype"
property. Whenever such a Function instance is created another ordinary
object is also created and is the initial value of the function's
"prototype" property. Unless otherwise specified, the value of the
"prototype" property is used to initialize the [[Prototype]] internal slot of the object created when that function
is invoked as a constructor.
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
The host-defined abstract operation
HostHasSourceTextAvailable takes argument func (a function object) and returns a
Boolean. It allows host environments to prevent the source text
from being provided for func.
An implementation of HostHasSourceTextAvailable must conform to the following requirements:
It must be deterministic with respect to its parameters. Each time it is called with a
specific func as its argument, it must return the same result.
The default implementation of HostHasSourceTextAvailable is to return true.
is the initial value of the "Boolean" property of the global
object.
creates and initializes a new Boolean object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Boolean behaviour must include a super call to the Boolean constructor to
create and initialize the subclass instance with a [[BooleanData]]
internal slot.
20.3.1.1 Boolean ( value )
This function performs the following steps when called:
Boolean instances are ordinary objects that inherit properties from the
Boolean prototype
object. Boolean instances have a [[BooleanData]]
internal slot. The [[BooleanData]] internal slot is the Boolean value
represented by this Boolean object.
The GlobalSymbolRegistry List is an append-only List that is globally
available. It is shared by all realms. Prior to the evaluation of any ECMAScript code,
it is initialized as a new empty List.
Elements of the GlobalSymbolRegistry List are Records with the
structure defined in Table
63.
The initial value of Symbol.prototype.constructor is %Symbol%.
20.4.3.2 get Symbol.prototype.description
Symbol.prototype.description is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
The initial value of the %Symbol.toStringTag% property is the
String value "Symbol".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
20.4.4 Properties of Symbol Instances
Symbol instances are ordinary objects that inherit properties from the
Symbol prototype
object. Symbol instances have a [[SymbolData]]
internal slot. The [[SymbolData]] internal slot is the Symbol value
represented by this Symbol object.
20.4.5 Abstract Operations for Symbols
20.4.5.1 KeyForSymbol ( sym )
The abstract operation KeyForSymbol takes argument sym (a Symbol) and returns a
String or undefined. If sym is in the GlobalSymbolRegistry
List, the String used to register sym will be returned. It
performs the following steps when called:
Instances of Error objects are thrown as exceptions when runtime errors occur. The Error objects may
also serve as base objects for user-defined exception classes.
When an ECMAScript implementation detects a runtime error, it throws a new instance of one of the
NativeError objects defined in 20.5.5 or a new instance
of the AggregateError object defined in 20.5.7.
is the initial value of the "Error" property of the global
object.
creates and initializes a new Error object when called as a function rather than as a
constructor. Thus the function call
Error(…) is equivalent to the object creation expression
new Error(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Error behaviour must include a super call to the Error constructor to
create and initialize subclass instances with an [[ErrorData]]
internal slot.
20.5.1.1 Error ( message [ , options ] )
This function performs the following steps when called:
If NewTarget is undefined, let newTarget be the
active function object; else
let newTarget be NewTarget.
If msg is undefined, set msg to the empty
String; otherwise set msg to ? ToString(msg).
If name is the empty String, return msg.
If msg is the empty String, return name.
Return the string-concatenation of
name, the code unit 0x003A (COLON), the code unit 0x0020 (SPACE), and
msg.
20.5.4 Properties of Error Instances
Error instances are ordinary objects that inherit properties from the
Error prototype
object and have an [[ErrorData]] internal slot
whose value is undefined. The only specified use of [[ErrorData]] is to identify Error, AggregateError, and
NativeError instances as Error objects within Object.prototype.toString
and Error.isError.
20.5.5 Native Error Types Used in This Standard
A new instance of one of the NativeError objects below or of the AggregateError object
is thrown when a runtime error is detected. All NativeError objects share the same
structure, as described in 20.5.6.
Indicates that one of the global URI handling functions was used in a way that is
incompatible with its definition.
20.5.6NativeError Object Structure
Each of these objects has the structure described below, differing only in the name used as the
constructor
name and in the "name" property of the prototype object.
For each error object, references to NativeError in the definition should be replaced
with the appropriate error object name from 20.5.5.
creates and initializes a new NativeError object when called as a function
rather than as a constructor. A call of the object as a
function is equivalent to calling it as a constructor with the same
arguments. Thus the function call NativeError(…) is equivalent
to the object creation expression new NativeError(…) with the
same arguments.
may be used as the value of an extends clause of a class definition.
Subclass constructors that intend to inherit the
specified NativeError behaviour must include a super call to the
NativeErrorconstructor to create and initialize subclass
instances with an [[ErrorData]] internal slot.
20.5.6.1.1NativeError ( message [ ,
options ] )
Each NativeError function performs the following steps when called:
If NewTarget is undefined, let newTarget be the
active function object;
else let newTarget be NewTarget.
The actual value of the string passed in step 2 is
either "%EvalError.prototype%",
"%RangeError.prototype%",
"%ReferenceError.prototype%",
"%SyntaxError.prototype%",
"%TypeError.prototype%", or "%URIError.prototype%"
corresponding to which NativeErrorconstructor is being defined.
20.5.6.2 Properties of the NativeError Constructors
The initial value of the "constructor" property of the prototype for a
given NativeErrorconstructor is the constructor
itself.
20.5.6.3.2NativeError.prototype.message
The initial value of the "message" property of the prototype for a
given NativeErrorconstructor is the empty String.
20.5.6.3.3NativeError.prototype.name
The initial value of the "name" property of the prototype for a given
NativeErrorconstructor is the String value consisting of
the name of the constructor (the name used instead of
NativeError).
20.5.6.4 Properties of NativeError Instances
NativeError instances are ordinary objects that inherit properties from
their NativeError prototype object and have an [[ErrorData]] internal slot whose value is
undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString
(20.1.3.6) and
Error.isError (20.5.2.1) to identify Error,
AggregateError, or NativeError instances.
is the initial value of the "AggregateError" property of the
global
object.
creates and initializes a new AggregateError object when called as a function rather
than as a constructor. Thus the function call
AggregateError(…) is equivalent to the object creation expression
new AggregateError(…) with the same arguments.
may be used as the value of an extends clause of a class definition.
Subclass constructors that intend to inherit the
specified AggregateError behaviour must include a super call to the
AggregateError constructor to create and initialize subclass
instances with an [[ErrorData]] internal slot.
The initial value of AggregateError.prototype.constructor is %AggregateError%.
20.5.7.3.2 AggregateError.prototype.message
The initial value of AggregateError.prototype.message is the empty String.
20.5.7.3.3 AggregateError.prototype.name
The initial value of AggregateError.prototype.name is
"AggregateError".
20.5.7.4 Properties of AggregateError Instances
AggregateError instances are ordinary objects that inherit properties from
their AggregateError
prototype object and have an [[ErrorData]]
internal slot whose value is undefined. The only specified use of [[ErrorData]] is by Object.prototype.toString
(20.1.3.6) and
Error.isError (20.5.2.1) to identify Error,
AggregateError, or NativeError instances.
20.5.8 Abstract Operations for Error Objects
20.5.8.1 InstallErrorCause ( O, options )
The abstract operation InstallErrorCause takes arguments O (an Object) and
options (an ECMAScript language value) and
returns either a normal completion
containingunused or a throw completion. It
is used to create a "cause" property on O when a
"cause" property is present on options. It performs the
following steps when called:
is the initial value of the "Number" property of the global
object.
creates and initializes a new Number object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Number behaviour must include a super call to the Number constructor to
create and initialize the subclass instance with a [[NumberData]]
internal slot.
21.1.1.1 Number ( value )
This function performs the following steps when called:
The value of Number.EPSILON is the Number value for the magnitude
of the difference between 1 and the smallest value greater than 1 that is representable as a
Number value, which is approximately 2.2204460492503130808472633361816 × 10-16.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.2 Number.isFinite ( number )
This function performs the following steps when called:
This function differs from the global isNaN function (19.2.3) in that it does not convert
its argument to a Number before determining whether it is NaN.
Due to rounding behaviour necessitated by precision limitations of IEEE
754-2019, the Number value for every
integer
greater than Number.MAX_SAFE_INTEGER is shared with at least one other
integer.
Such large-magnitude integers are therefore not safe,
and are not guaranteed to be exactly representable as Number values or even to be
distinguishable from each other. For example, both 9007199254740992 and
9007199254740993 evaluate to the Number value
9007199254740992𝔽.
The value of Number.MAX_SAFE_INTEGER is
9007199254740991𝔽 (𝔽(253 - 1)).
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.7 Number.MAX_VALUE
The value of Number.MAX_VALUE is the largest positive finite value of the Number type, which
is approximately 1.7976931348623157 × 10308.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.8 Number.MIN_SAFE_INTEGER
Note
Due to rounding behaviour necessitated by precision limitations of IEEE
754-2019, the Number value for every
integer
less than Number.MIN_SAFE_INTEGER is shared with at least one other
integer.
Such large-magnitude integers are therefore not safe,
and are not guaranteed to be exactly representable as Number values or even to be
distinguishable from each other. For example, both -9007199254740992
and -9007199254740993 evaluate to the Number value
-9007199254740992𝔽.
The value of Number.MIN_SAFE_INTEGER is
-9007199254740991𝔽 (𝔽(-(253 - 1))).
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.9 Number.MIN_VALUE
The value of Number.MIN_VALUE is the smallest positive value of the Number type, which
is approximately 5 × 10-324.
In the IEEE
754-2019 double precision binary representation, the smallest
possible value is a denormalized number. If an implementation does not support denormalized
values, the value of Number.MIN_VALUE must be the smallest non-zero positive
value that can actually be represented by the implementation.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.10 Number.NaN
The value of Number.NaN is NaN.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.11 Number.NEGATIVE_INFINITY
The value of Number.NEGATIVE_INFINITY is -∞𝔽.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.1.2.12 Number.parseFloat ( string )
The initial value of the "parseFloat" property is %parseFloat%.
21.1.2.13 Number.parseInt ( string, radix
)
The initial value of the "parseInt" property is %parseInt%.
21.1.2.14 Number.POSITIVE_INFINITY
The value of Number.POSITIVE_INFINITY is +∞𝔽.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
Unless explicitly stated otherwise, the methods of the Number prototype object defined below are
not generic and the this value passed to them must be either a Number value
or an object that has a [[NumberData]] internal slot that has been
initialized to a Number value.
The phrase “this Number value” within the specification of a method refers to the result returned
by calling the abstract operation ThisNumberValue with the
this value of the method invocation passed as the argument.
21.1.3.1 Number.prototype.constructor
The initial value of Number.prototype.constructor is %Number%.
This method returns a String containing this Number value represented in decimal exponential
notation with one digit before the significand's decimal point and fractionDigits
digits after the significand's decimal point. If fractionDigits is
undefined, it includes as many significand digits as necessary to
uniquely specify the Number (just like in ToString except that in this case the Number is
always output in exponential notation).
Let m be the String value consisting of f + 1
occurrences of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else,
If fractionDigits is not undefined, then
Let e and n be integers such
that 10f ≤ n <
10f + 1 and for which n ×
10e - f - x is as close
to zero as possible. If there are two such sets of e and
n, pick the e and n for which
n × 10e - f is larger.
Else,
Let
e, n, and ff be integers such that
ff ≥ 0, 10ff ≤ n <
10ff + 1, 𝔽(n ×
10e - ff) is 𝔽(x),
and ff is as small as possible. Note that the decimal
representation of n has ff + 1 digits,
n is not divisible by 10, and the least significant digit
of n is not necessarily uniquely determined by these
criteria.
Set f to ff.
Let m be the String value consisting of the digits of the decimal
representation of n (in order, with no leading zeroes).
For implementations that provide more accurate conversions than required by the rules
above, it is recommended that the following alternative version of step 10.b.i
be used as a guideline:
Let e, n, and f be integers such that f ≥ 0,
10f ≤ n < 10f + 1,
𝔽(n × 10e -
f) is 𝔽(x), and f is as
small as possible. If there are multiple possibilities for n,
choose the value of n for which 𝔽(n ×
10e - f) is closest in value to 𝔽(x).
If there are two such possible values of n, choose the one that
is even.
This method returns a String containing this Number value represented in decimal
fixed-point notation with fractionDigits digits after the decimal point.
If fractionDigits is undefined, 0 is assumed.
Let n be an integer for which n /
10f - x is as close to zero as possible. If
there are two such n, pick the larger n.
If n = 0, let m be "0". Otherwise,
let m be the String value consisting of the digits of the decimal
representation of n (in order, with no leading zeroes).
If f ≠ 0, then
Let k be the length of m.
If k ≤ f, then
Let z be the String value consisting of
f + 1 - k occurrences of the code unit
0x0030 (DIGIT ZERO).
The output of toFixed may be more precise than toString for
some values because toString only prints enough significant digits to distinguish
the number from adjacent Number values. For example,
(1000000000000000128).toString() returns
"1000000000000000100", while (1000000000000000128).toFixed(0) returns
"1000000000000000128".
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method produces a String value that represents this Number value formatted according to
the conventions of the host environment's current locale. This
method is implementation-defined, and it is
permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
This method returns a String containing this Number value represented either in decimal
exponential notation with one digit before the significand's decimal point and precision - 1 digits after the significand's decimal
point or in decimal fixed notation with precision significant digits. If
precision is undefined, it calls ToString instead.
Let m be the String value consisting of p occurrences
of the code unit 0x0030 (DIGIT ZERO).
Let e be 0.
Else,
Let e and n be integers such that
10p - 1 ≤ n < 10p
and for which n × 10e - p + 1 -
x is as close to zero as possible. If there are two such sets of
e and n, pick the e and n for
which n × 10e - p + 1 is larger.
Let m be the String value consisting of the digits of the decimal
representation of n (in order, with no leading zeroes).
Set m to the string-concatenation of
the first e + 1 code units of m, the code unit 0x002E
(FULL STOP), and the remaining p - (e + 1) code units
of m.
Else,
Set m to the string-concatenation of
the code unit 0x0030 (DIGIT ZERO), the code unit 0x002E (FULL STOP),
-(e + 1) occurrences of the code unit 0x0030 (DIGIT ZERO), and
the String m.
The optional radix should be an integral Number value
in the inclusive interval from
2𝔽 to 36𝔽. If
radix is undefined then
10𝔽 is used as the value of radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its
this value is not a
Number or a Number object. Therefore, it cannot be transferred to
other kinds of objects for use as a method.
Number instances are ordinary objects that inherit properties from the
Number prototype
object. Number instances also have a [[NumberData]] internal slot. The [[NumberData]] internal slot is the Number value represented by this
Number object.
is the initial value of the "BigInt" property of the global
object.
performs a type conversion when called as a function rather than as a constructor.
is not intended to be used with the new operator or to be subclassed. It may be
used as the value of an extends clause of a class definition but a
super call to the BigInt constructor will cause an exception.
21.2.1.1 BigInt ( value )
This function performs the following steps when called:
If NewTarget is not undefined, throw a
TypeError exception.
The abstract operation NumberToBigInt takes argument number (a Number) and
returns either a normal completion
containing a BigInt or a throw
completion. It performs the following steps when called:
If number is not an integral Number,
throw a RangeError exception.
The phrase “this BigInt value” within the specification of a method refers to the result returned
by calling the abstract operation ThisBigIntValue with the
this value of the method invocation passed as the argument.
21.2.3.1 BigInt.prototype.constructor
The initial value of BigInt.prototype.constructor is %BigInt%.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method produces a String value that represents this BigInt value formatted according to
the conventions of the host environment's current locale. This
method is implementation-defined, and it is
permissible, but not encouraged, for it to return the same thing as toString.
The meanings of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
21.2.3.3 BigInt.prototype.toString ( [ radix ] )
Note
The optional radix should be an integral Number value
in the inclusive interval from
2𝔽 to 36𝔽. If
radix is undefined then
10𝔽 is used as the value of radix.
This method performs the following steps when called:
This method is not generic; it throws a TypeError exception if its
this value is not a
BigInt or a BigInt object. Therefore, it cannot be transferred to
other kinds of objects for use as a method.
The initial value of the %Symbol.toStringTag% property is the
String value "BigInt".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
21.2.4 Properties of BigInt Instances
BigInt instances are ordinary objects that inherit properties from the
BigInt prototype
object. BigInt instances also have a [[BigIntData]] internal slot. The [[BigIntData]] internal slot is the BigInt value represented by this
BigInt object.
21.3 The Math Object
The Math object:
is %Math%.
is the initial value of the "Math" property of the global
object.
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
Note
In this specification, the phrase “the Number value forx” has a
technical meaning defined in 6.1.6.1.
21.3.1 Value Properties of the Math Object
21.3.1.1 Math.E
The Number
value fore, the base of the natural logarithms, which is
approximately 2.7182818284590452354.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.2 Math.LN10
The Number
value for the natural logarithm of 10, which is approximately
2.302585092994046.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.3 Math.LN2
The Number
value for the natural logarithm of 2, which is approximately
0.6931471805599453.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.4 Math.LOG10E
The Number
value for the base-10 logarithm of e, the base of the natural
logarithms; this value is approximately 0.4342944819032518.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
The value of Math.LOG10E is approximately the reciprocal of the value of
Math.LN10.
21.3.1.5 Math.LOG2E
The Number
value for the base-2 logarithm of e, the base of the natural
logarithms; this value is approximately 1.4426950408889634.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
The value of Math.LOG2E is approximately the reciprocal of the value of
Math.LN2.
21.3.1.6 Math.PI
The Number
value for π, the ratio of the circumference of a circle to its
diameter, which is approximately 3.1415926535897932.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.7 Math.SQRT1_2
The Number
value for the square root of ½, which is approximately
0.7071067811865476.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
The value of Math.SQRT1_2 is approximately the reciprocal of the value
of Math.SQRT2.
21.3.1.8 Math.SQRT2
The Number
value for the square root of 2, which is approximately
1.4142135623730951.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
21.3.1.9 Math [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the
String value "Math".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
21.3.2 Function Properties of the Math Object
Note
The behaviour of the functions acos, acosh, asin,
asinh, atan, atanh, atan2,
cbrt, cos, cosh, exp,
expm1, hypot, log, log1p,
log2, log10, pow, random,
sin, sinh, tan, and tanh is not
precisely specified here except to require specific results for certain argument values
that represent boundary cases of interest. For other argument values, these functions
are intended to compute approximations to the results of familiar mathematical
functions, but some latitude is allowed in the choice of approximation algorithms. The
general intent is that an implementer should be able to use the same mathematical
library for ECMAScript on a given hardware platform that is available to C programmers
on that platform.
Although the choice of algorithms is left to the implementation, it is recommended (but
not specified by this standard) that implementations use the approximation algorithms
for IEEE
754-2019 arithmetic contained in fdlibm, the freely
distributable mathematical library from Sun Microsystems (http://www.netlib.org/fdlibm).
21.3.2.1 Math.abs ( x )
This function returns the absolute value of x; the result has the same magnitude
as x but has positive sign.
This function returns the inverse tangent of the quotient y / x of the arguments y and
x, where the signs of y and x are used to determine the
quadrant of the result. Note that it is intentional and traditional for the two-argument
inverse tangent function that the argument named y be first and the argument
named x be second. The result is expressed in radians and is in the inclusive
interval from -π to +π.
This function returns the result of subtracting 1 from the exponential function of
x (e raised to the power of x, where e is the
base of the natural logarithms). The result is computed in a way that is accurate even when
the value of x is close to 0.
This function returns the greatest (closest to +∞) integral Number value that is
not greater than x. If x is already an integral
Number, the result is x.
Let n16 be the result of converting n to IEEE
754-2019 binary16 format using roundTiesToEven mode.
Let n64 be the result of converting n16 to IEEE
754-2019 binary64 format.
Return the ECMAScript Number value corresponding to n64.
Note
This operation is not the same as casting to binary32 and then to binary16 because of
the possibility of double-rounding: consider the number k =
1.00048828125000022204𝔽, for example, for which
Math.f16round(k) is 1.0009765625𝔽, but
Math.f16round(Math.fround(k)) is 1𝔽.
Not all platforms provide native support for casting from binary64 to binary16. There
are various libraries which can provide this, including the MIT-licensed half library. Alternatively, it is
possible to first cast from binary64 to binary32 under roundTiesToEven and then
check whether the result could lead to incorrect double-rounding. The cases which
could can be handled explicitly by adjusting the mantissa of the binary32 value so
that it is the value which would be produced by performing the initial cast under
roundTiesToOdd. Casting the adjusted value to binary16 under roundTiesToEven then
produces the correct value.
21.3.2.19 Math.hypot ( ...args )
Given zero or more arguments, this function returns the square root of the sum of squares of
its arguments.
Implementations should take care to avoid the loss of precision from overflows and
underflows that are prone to occur in naive implementations when this function is
called with two or more arguments.
21.3.2.20 Math.imul ( x, y )
This function performs the following steps when called:
This function returns a Number value with positive sign, greater than or equal to
+0𝔽 but strictly less than 1𝔽,
chosen randomly or pseudo randomly with approximately uniform distribution over that range,
using an implementation-defined algorithm or
strategy.
Each Math.random function created for distinct realms must produce a distinct sequence
of values from successive calls.
21.3.2.29 Math.round ( x )
This function returns the Number value that is closest to x and is integral. If
two integral
Numbers are equally close to x, then the result is the
Number value that is closer to +∞. If x is already integral, the result is
x.
Return the integral Number closest to
n, preferring the Number closer to +∞ in the case of a tie.
Note 1
Math.round(3.5) returns 4, but Math.round(-3.5) returns -3.
Note 2
The value of Math.round(x) is not always the same as the value of
Math.floor(x + 0.5). When x is
-0𝔽 or x is less than
-0𝔽 but greater than or equal to
-0.5𝔽, Math.round(x) returns
-0𝔽, but Math.floor(x + 0.5) returns
+0𝔽. Math.round(x) may also differ from
the value of Math.floor(x + 0.5)because of internal rounding when
computing x + 0.5.
21.3.2.30 Math.sign ( x )
This function returns the sign of x, indicating whether x is positive,
negative, or zero.
If n is not finite or n is either
+0𝔽 or -0𝔽, return
n.
If n < 1𝔽 and n >
+0𝔽, return +0𝔽.
If n < -0𝔽 and n >
-1𝔽, return -0𝔽.
Return the integral Number nearest n
in the direction of +0𝔽.
21.4 Date Objects
21.4.1 Overview of Date Objects and Definitions of Abstract
Operations
The following abstract operations
operate on time values (defined in 21.4.1.1). Note that, in every
case, if any argument to one of these functions is NaN, the result will be
NaN.
21.4.1.1 Time Values and Time Range
Time measurement in ECMAScript is analogous to time measurement in POSIX, in particular
sharing definition in terms of the proleptic Gregorian calendar, an epoch of midnight at the beginning of 1 January 1970 UTC, and an
accounting of every day as comprising exactly 86,400 seconds (each of which is 1000
milliseconds long).
An ECMAScript time value is a Number, either
a finiteintegral
Number representing an instant in time to millisecond precision or
NaN representing no specific instant. A time value that is a multiple of
24 × 60 × 60 × 1000 = 86,400,000 (i.e., is 86,400,000 ×
d for some integerd) represents the instant at the
start of the UTC day that follows the epoch by d whole UTC days (preceding the
epoch for negative
d). Every other finite time value t is defined relative to
the greatest preceding time value s that is such a multiple, and represents the
instant that occurs within the same UTC day as s but follows it by (t
- s) milliseconds.
Time values do not account for UTC leap seconds—there are no time values representing
instants within positive leap seconds, and there are time values representing instants
removed from the UTC timeline by negative leap seconds. However, the definition of time
values nonetheless yields piecewise alignment with UTC, with discontinuities only at leap
second boundaries and zero difference outside of leap seconds.
A Number can exactly represent all integers from -9,007,199,254,740,992 to
9,007,199,254,740,992 (21.1.2.8 and 21.1.2.6). A time value supports
a slightly smaller range of -8,640,000,000,000,000 to 8,640,000,000,000,000 milliseconds.
This yields a supported time value range of exactly -100,000,000 days to 100,000,000 days
relative to midnight at the beginning of 1 January 1970 UTC.
The exact moment of midnight at the beginning of 1 January 1970 UTC is represented by the
time value +0𝔽.
Note
In the proleptic Gregorian calendar, leap years are precisely those which are both
divisible by 4 and either divisible by 400 or not divisible by 100.
The 400 year cycle of the proleptic Gregorian calendar contains 97 leap years. This
yields an average of 365.2425 days per year, which is 31,556,952,000 milliseconds.
Therefore, the maximum range a Number could represent exactly with millisecond
precision is approximately -285,426 to 285,426 years relative to 1970. The smaller
range supported by a time value as specified in this section is approximately
-273,790 to 273,790 years relative to 1970.
21.4.1.2 Time-related Constants
These constants are referenced by algorithms in the following sections.
The abstract operation Day takes argument t (a finitetime value) and returns an
integral
Number. It returns the day number of the day in which t
falls. It performs the following steps when called:
The abstract operation TimeWithinDay takes argument t (a finitetime value) and returns an
integral
Number in the interval from +0𝔽
(inclusive) to msPerDay (exclusive). It returns the number of
milliseconds since the start of the day in which t falls. It performs the
following steps when called:
The abstract operation DaysInYear takes argument y (an integral
Number) and returns 365𝔽 or
366𝔽. It returns the number of days in year y.
Leap years have 366 days; all other years have 365. It performs the following steps when
called:
The abstract operation DayFromYear takes argument y (an integral
Number) and returns an integral Number. It returns
the day number of the first day of year y. It performs the following steps when
called:
NOTE: In the following steps,
numYears1, numYears4, numYears100, and
numYears400 represent the number of years divisible by 1, 4, 100, and
400, respectively, that occur between the epoch and the start of year y. The
number is negative if y is before the epoch.
The abstract operation TimeFromYear takes argument y (an integral
Number) and returns a time value. It returns the
time value of the start of
year y. It performs the following steps when called:
The abstract operation YearFromTime takes argument t (a finitetime value) and returns an
integral
Number. It returns the year in which t falls. It performs
the following steps when called:
The abstract operation InLeapYear takes argument t (a finitetime value) and returns
+0𝔽 or 1𝔽. It returns
1𝔽 if t is within a leap year and
+0𝔽 otherwise. It performs the following steps when called:
The abstract operation MonthFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from
+0𝔽 to 11𝔽. It returns a
Number identifying the month in which t falls. A month value of
+0𝔽 specifies January; 1𝔽
specifies February; 2𝔽 specifies March;
3𝔽 specifies April; 4𝔽
specifies May; 5𝔽 specifies June;
6𝔽 specifies July; 7𝔽
specifies August; 8𝔽 specifies September;
9𝔽 specifies October; 10𝔽
specifies November; and 11𝔽 specifies December. Note that
MonthFromTime(+0𝔽) =
+0𝔽, corresponding to Thursday, 1 January 1970.
It performs the following steps when called:
The abstract operation DateFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from
1𝔽 to 31𝔽. It returns the day
of the month in which t falls. It performs the following steps when called:
The abstract operation WeekDay takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from
+0𝔽 to 6𝔽. It returns a Number
identifying the day of the week in which t falls. A weekday value of
+0𝔽 specifies Sunday; 1𝔽
specifies Monday; 2𝔽 specifies Tuesday;
3𝔽 specifies Wednesday; 4𝔽
specifies Thursday; 5𝔽 specifies Friday; and
6𝔽 specifies Saturday. Note that WeekDay(+0𝔽) =
4𝔽, corresponding to Thursday, 1 January 1970.
It performs the following steps when called:
The abstract operation HourFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from
+0𝔽 to 23𝔽. It returns the
hour of the day in which t falls. It performs the following steps when called:
The abstract operation MinFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from
+0𝔽 to 59𝔽. It returns the
minute of the hour in which t falls. It performs the following steps when called:
The abstract operation SecFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from
+0𝔽 to 59𝔽. It returns the
second of the minute in which t falls. It performs the following steps when
called:
The abstract operation msFromTime takes argument t (a finitetime value) and returns an
integral
Number in the inclusive interval from
+0𝔽 to 999𝔽. It returns the
millisecond of the second in which t falls. It performs the following steps when
called:
Time zones in ECMAScript are represented by time zone identifiers, which are Strings composed entirely of code
units in the inclusive interval from 0x0000 to 0x007F.
Time zones supported by an ECMAScript implementation may be available named time zones,
represented by the [[Identifier]] field of the Time Zone Identifier Records
returned by AvailableNamedTimeZoneIdentifiers,
or offset time zones, represented by
Strings for which IsTimeZoneOffsetString returns
true.
A primary time zone
identifier is the preferred identifier for an available named time zone.
A non-primary time zone
identifier is an identifier for an available named time zone that is not a primary
time zone identifier.
An available named time
zone identifier is either a primary time zone identifier or a non-primary time
zone identifier.
Each available named time zone identifier is associated with exactly one available named
time zone.
Each available named time zone is associated with exactly one primary time zone identifier
and zero or more non-primary time zone identifiers.
ECMAScript implementations must support an available named time zone with the identifier
"UTC", which must be the primary time zone identifier for the UTC time
zone.
In addition, implementations may support any number of other available named time zones.
Implementations that follow the requirements for time zones as described in the ECMA-402
Internationalization API specification are called time zone aware.
Time zone aware implementations must support available named time zones corresponding to the
Zone and Link names of the IANA Time Zone Database, and only such names.
In time zone aware implementations, a primary time zone identifier is a Zone name, and a
non-primary time zone identifier is a Link name, respectively, in the IANA Time Zone
Database except as specifically overridden by AvailableNamedTimeZoneIdentifiers
as specified in the ECMA-402 specification.
Implementations that do not support the entire IANA Time Zone Database are still recommended
to use IANA Time Zone Database names as identifiers to represent time zones.
When the input represents a local time occurring more than once because of a negative time
zone transition (e.g. when daylight saving time ends or the time zone offset is decreased
due to a time zone rule change), the returned List will have more
than one element and will be sorted by ascending numerical value.
When the input represents a local time skipped because of a positive time zone transition
(e.g. when daylight saving time begins or the time zone offset is increased due to a time
zone rule change), the returned List
will be empty.
Otherwise, the returned List
will have one element.
The default implementation of GetNamedTimeZoneEpochNanoseconds, to be used for ECMAScript
implementations that do not include local political rules for any time zones, performs the
following steps when called:
1:30 AM on 5 November 2017 in America/New_York is repeated twice, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 11, 5,
1, 30, 0, 0, 0, 0) would return a List of length
2 in which the first element represents 05:30 UTC (corresponding with 01:30 US
Eastern Daylight Time at UTC offset -04:00) and the second element represents 06:30
UTC (corresponding with 01:30 US Eastern Standard Time at UTC offset -05:00).
2:30 AM on 12 March 2017 in America/New_York does not exist, so
GetNamedTimeZoneEpochNanoseconds("America/New_York", 2017, 3, 12,
2, 30, 0, 0, 0, 0) would return an empty List.
The implementation-defined abstract
operation GetNamedTimeZoneOffsetNanoseconds takes arguments timeZoneIdentifier (a
String) and epochNanoseconds (a BigInt) and returns an integer.
The returned integer represents the offset from UTC of the named
time zone identified by timeZoneIdentifier, at the instant corresponding with
epochNanoseconds relative to the epoch, both in nanoseconds.
The default implementation of GetNamedTimeZoneOffsetNanoseconds, to be used for ECMAScript
implementations that do not include local political rules for any time zones, performs the
following steps when called:
Time zone aware implementations,
including all implementations that implement the ECMA-402 Internationalization API, must
implement the AvailableNamedTimeZoneIdentifiers abstract operation as specified in the
ECMA-402 specification.
For implementations that are not time zone aware,
AvailableNamedTimeZoneIdentifiers performs the following steps when called:
If the implementation does not include local political rules for any time zones,
then
The implementation-defined abstract
operation SystemTimeZoneIdentifier takes no arguments and returns a String.
It returns a String representing the host environment's current time zone, which
is either a String representing a UTC offset for which IsTimeZoneOffsetString returns
true, or a primary time zone identifier.
It performs the following steps when called:
If the implementation only supports the UTC time zone, return
"UTC".
To ensure the level of functionality that implementations commonly provide in the
methods of the Date object, it is recommended that SystemTimeZoneIdentifier return
an IANA time zone name corresponding to the host environment's
time zone setting, if such a thing exists.
GetNamedTimeZoneEpochNanoseconds
and GetNamedTimeZoneOffsetNanoseconds
must reflect the local political rules for standard time and daylight saving time in
that time zone, if such rules exist.
For example, if the host environment is a browser on a
system where the user has chosen US Eastern Time as their time zone,
SystemTimeZoneIdentifier returns "America/New_York".
21.4.1.25 LocalTime ( t )
The abstract operation LocalTime takes argument t (a finitetime value) and returns an
integral
Number.
It converts t from UTC to local time.
The local political rules for standard time and daylight saving time in effect at
t should be used to determine the result in the way specified in this section.
It performs the following steps when called:
Two different input time
valuestUTC are converted to the same
local time tlocal at a negative time
zone transition when there are repeated times (e.g. the daylight saving time ends or
the time zone adjustment is decreased.).
LocalTime(UTC(tlocal))
is not necessarily always equal to tlocal. Correspondingly, UTC(LocalTime(tUTC))
is not necessarily always equal to tUTC.
21.4.1.26 UTC ( t )
The abstract operation UTC takes argument t (a Number) and returns a time value.
It converts t from local time to a UTC time value.
The local political rules for standard time and daylight saving time in effect at
t should be used to determine the result in the way specified in this section.
It performs the following steps when called:
NOTE: The following steps ensure that when t represents local
time repeating multiple times at a negative time zone transition (e.g. when
the daylight saving time ends or the time zone offset is decreased due to a
time zone rule change) or skipped local time at a positive time zone
transition (e.g. when the daylight saving time starts or the time zone
offset is increased due to a time zone rule change), t is
interpreted using the time zone offset before the transition.
If possibleInstants is not empty, then
Let disambiguatedInstant be
possibleInstants[0].
Else,
NOTE: t represents a local time skipped at a positive
time zone transition (e.g. due to daylight saving time starting or a
time zone rule change increasing the UTC offset).
Input t is nominally a time value but
may be any Number value.
The algorithm must not limit t to the time value range, so that
inputs corresponding with a boundary of the time value
range can be supported regardless of local UTC offset.
For example, the maximum time value is 8.64 ×
1015, corresponding with "+275760-09-13T00:00:00Z".
In an environment where the local time zone offset is ahead of UTC by 1 hour at that
instant, it is represented by the larger input of 8.64 × 1015 + 3.6 ×
106, corresponding with "+275760-09-13T01:00:00+01:00".
1:30 AM on 5 November 2017 in America/New_York is repeated twice (fall backward),
but it must be interpreted as 1:30 AM UTC-04 instead of 1:30 AM UTC-05.
In UTC(TimeClip(MakeDate(MakeDay(2017, 10, 5), MakeTime(1, 30, 0, 0)))), the value of
offsetMs is -4 × msPerHour.
2:30 AM on 12 March 2017 in America/New_York does not exist, but it must be
interpreted as 2:30 AM UTC-05 (equivalent to 3:30 AM UTC-04).
In UTC(TimeClip(MakeDate(MakeDay(2017, 2, 12), MakeTime(2, 30, 0, 0)))), the value of
offsetMs is -5 × msPerHour.
Note 2
UTC(LocalTime(tUTC))
is not necessarily always equal to tUTC. Correspondingly, LocalTime(UTC(tlocal))
is not necessarily always equal to tlocal.
21.4.1.27 MakeTime ( hour, min,
sec, ms )
The abstract operation MakeTime takes arguments hour (a Number), min (a
Number), sec (a Number), and ms (a Number) and returns a Number. It
calculates a number of milliseconds. It performs the following steps when called:
The arithmetic in MakeTime is floating-point arithmetic, which is not associative, so
the operations must be performed in the correct order.
21.4.1.28 MakeDay ( year, month,
date )
The abstract operation MakeDay takes arguments year (a Number), month
(a Number), and date (a Number) and returns a Number. It calculates a number of
days. It performs the following steps when called:
If year is not finite, month is not finite, or
date is not finite, return NaN.
The abstract operation MakeDate takes arguments day (a Number) and time
(a Number) and returns a Number. It calculates a number of milliseconds. It performs the
following steps when called:
If day is not finite or time is not finite, return
NaN.
The abstract operation MakeFullYear takes argument year (a Number) and returns an
integral
Number or NaN. It returns the full year associated
with the integer
part of year, interpreting any value in the inclusive interval from 0
to 99 as a count of years since the start of 1900. For alignment with the proleptic
Gregorian calendar, "full year" is defined as the signed count of complete years since the
start of year 0 (1 B.C.). It performs the following steps when called:
The abstract operation TimeClip takes argument time (a Number) and returns a
Number. It calculates a number of milliseconds. It performs the following steps when called:
ECMAScript defines a string interchange format for date-times based upon a simplification of
the ISO 8601 calendar date extended format. The format is as follows:
YYYY-MM-DDTHH:mm:ss.sssZ
Where the elements are as follows:
YYYY
is the year in the proleptic Gregorian calendar as four decimal digits from
0000 to 9999, or as an expanded year of
"+" or "-" followed by six decimal
digits.
-
"-" (hyphen) appears literally twice in the string.
MM
is the month of the year as two decimal digits from 01 (January) to 12
(December).
DD
is the day of the month as two decimal digits from 01 to 31.
T
"T" appears literally in the string, to indicate the
beginning of the time element.
HH
is the number of complete hours that have passed since midnight as two
decimal digits from 00 to 24.
:
":" (colon) appears literally twice in the string.
mm
is the number of complete minutes since the start of the hour as two decimal
digits from 00 to 59.
ss
is the number of complete seconds since the start of the minute as two
decimal digits from 00 to 59.
.
"." (dot) appears literally in the string.
sss
is the number of complete milliseconds since the start of the second as
three decimal digits.
Z
is the UTC offset representation specified as "Z" (for
UTC with no offset) or as either "+" or
"-" followed by a time expression HH:mm (a
subset of the time zone offset
string format for indicating local time ahead of or
behind UTC, respectively)
This format includes date-only forms:
YYYY
YYYY-MM
YYYY-MM-DD
It also includes “date-time” forms that consist of one of the above date-only forms
immediately followed by one of the following time forms with an optional UTC offset
representation appended:
THH:mm
THH:mm:ss
THH:mm:ss.sss
A string containing out-of-bounds or nonconforming elements is not a valid instance of this
format.
Note 1
As every day both starts and ends with midnight, the two notations 00:00
and 24:00 are available to distinguish the two midnights that can be
associated with one date. This means that the following two notations refer to
exactly the same point in time: 1995-02-04T24:00 and
1995-02-05T00:00. This interpretation of the latter form as "end of a
calendar day" is consistent with ISO 8601, even though that specification reserves
it for describing time intervals and does not permit it within representations of
single points in time.
Note 2
There exists no international standard that specifies abbreviations for civil time
zones like CET, EST, etc. and sometimes the same abbreviation is even used for two
very different time zones. For this reason, both ISO 8601 and this format specify
numeric representations of time zone offsets.
21.4.1.32.1 Expanded Years
Covering the full time value range of
approximately 273,790 years forward or backward from 1 January 1970 (21.4.1.1) requires
representing years before 0 or after 9999. ISO 8601 permits expansion of the year
representation, but only by mutual agreement of the partners in information interchange.
In the simplified ECMAScript format, such an expanded year representation shall have 6
digits and is always prefixed with a + or - sign. The year 0 is considered positive and
must be prefixed with a + sign. The representation of the year 0 as -000000 is invalid.
Strings matching the Date Time String Format with
expanded years representing instants in time outside the range of a time value are treated as
unrecognizable by Date.parse and cause that
function to return NaN without falling back to
implementation-specific behaviour or heuristics.
Note
Examples of date-time values with expanded years:
-271821-04-20T00:00:00Z
271822 B.C.
-000001-01-01T00:00:00Z
2 B.C.
+000000-01-01T00:00:00Z
1 B.C.
+000001-01-01T00:00:00Z
1 A.D.
+001970-01-01T00:00:00Z
1970 A.D.
+002009-12-15T00:00:00Z
2009 A.D.
+275760-09-13T00:00:00Z
275760 A.D.
21.4.1.33 Time Zone Offset String Format
ECMAScript defines a string interchange format for UTC offsets, derived from ISO 8601.
The format is described by the following grammar.
The abstract operation IsTimeZoneOffsetString takes argument offsetString (a
String) and returns a Boolean. The return value indicates whether
offsetString conforms to the grammar given by UTCOffset. It performs the following steps
when called:
The abstract operation ParseTimeZoneOffsetString takes argument offsetString
(a String) and returns an integer. The return value is the UTC offset, as a
number of nanoseconds, that corresponds to the String offsetString. It
performs the following steps when called:
If parsedSign is the single code point U+002D (HYPHEN-MINUS), then
Let sign be -1.
Else,
Let sign be 1.
NOTE: Applications of StringToNumber below do not
lose precision, since each of the parsed values is guaranteed to be a
sufficiently short string of decimal digits.
is the initial value of the "Date" property of the global
object.
creates and initializes a new Date when called as a constructor.
returns a String representing the current time (UTC) when called as a function rather than
as a constructor.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Date behaviour must include a super call to the Date constructor to
create and initialize the subclass instance with a [[DateValue]]
internal slot.
21.4.2.1 Date ( ...values )
This function performs the following steps when called:
If NewTarget is undefined, then
Let now be the time
value (UTC) identifying the current time.
This function returns the time value designating the UTC
date and time of the occurrence of the call to it.
21.4.3.2 Date.parse ( string )
This function applies the ToString operator to its argument. If ToString results
in an abrupt completion
the Completion Record is
immediately returned. Otherwise, this function interprets the resulting String as a date and
time; it returns a Number, the UTC time value
corresponding to the date and time. The String may be interpreted as a local time, a UTC
time, or a time in some other time zone, depending on the contents of the String. The
function first attempts to parse the String according to the format described in Date Time
String Format (21.4.1.32), including expanded
years. If the String does not conform to that format the function may fall back to any
implementation-specific heuristics or implementation-specific date formats. Strings that are
unrecognizable or contain out-of-bounds format element values shall cause this function to
return NaN.
If the String conforms to the Date Time String Format,
substitute values take the place of absent format elements. When the MM or
DD elements are absent, "01" is used. When the
HH, mm, or ss elements are absent,
"00" is used. When the sss element is absent,
"000" is used. When the UTC offset representation is absent, date-only
forms are interpreted as a UTC time and date-time forms are interpreted as a local time.
If x is any Date whose milliseconds amount is zero within a particular
implementation of ECMAScript, then all of the following expressions should produce the same
numeric value in that implementation, if all the properties referenced have their initial
values:
is not required to produce the same Number value as the preceding three expressions and, in
general, the value produced by this function is implementation-defined
when given any String value that does not conform to the Date Time String Format (21.4.1.32) and that could not be
produced in that implementation by the toString or toUTCString
method.
This function differs from the Date constructor in two ways: it returns a
time value as a
Number, rather than creating a Date, and it interprets the arguments in UTC rather
than as local time.
Unless explicitly defined otherwise, the methods of the Date prototype object defined below are
not generic and the this value passed to them must be an object that has a
[[DateValue]] internal slot that has been initialized to a time value.
21.4.4.1 Date.prototype.constructor
The initial value of Date.prototype.constructor is %Date%.
21.4.4.2 Date.prototype.getDate ( )
This method performs the following steps when called:
If month is not present, this method behaves as if month was
present with the value getMonth(). If date is not present,
it behaves as if date was present with the value getDate().
21.4.4.22 Date.prototype.setHours ( hour [ ,
min [ , sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was
present with the value getMinutes(). If sec is not present,
it behaves as if sec was present with the value
getSeconds(). If ms is not present, it behaves as if
ms was present with the value getMilliseconds().
21.4.4.23 Date.prototype.setMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was
present with the value getSeconds(). If ms is not present,
this behaves as if ms was present with the value
getMilliseconds().
21.4.4.25 Date.prototype.setMonth ( month [ ,
date ] )
This method performs the following steps when called:
If month is not present, this method behaves as if month was
present with the value getUTCMonth(). If date is not
present, it behaves as if date was present with the value
getUTCDate().
21.4.4.30 Date.prototype.setUTCHours ( hour [ ,
min [ , sec [ , ms ] ] ] )
This method performs the following steps when called:
If min is not present, this method behaves as if min was
present with the value getUTCMinutes(). If sec is not
present, it behaves as if sec was present with the value
getUTCSeconds(). If ms is not present, it behaves as if
ms was present with the value getUTCMilliseconds().
21.4.4.31 Date.prototype.setUTCMilliseconds ( ms )
This method performs the following steps when called:
If sec is not present, this method behaves as if sec was
present with the value getUTCSeconds(). If ms is not
present, it behaves as if ms was present with the value return by
getUTCMilliseconds().
21.4.4.33 Date.prototype.setUTCMonth ( month [ ,
date ] )
This method performs the following steps when called:
If tv corresponds with a year that cannot be represented in the Date Time String Format,
throw a RangeError exception.
Return a String representation of tv in the Date Time String Format
on the UTC time scale, including all format elements and the UTC offset
representation "Z".
21.4.4.37 Date.prototype.toJSON ( key )
This method provides a String representation of a Date for use by JSON.stringify
(25.5.2).
This method is intentionally generic; it does not require that its
this value be a Date. Therefore, it can be transferred to other
kinds of objects for use as a method. However, it does require that any such object
have a toISOString method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method returns a String value. The contents of the String are implementation-defined, but are
intended to represent the “date” portion of the Date in the current time zone in a
convenient, human-readable form that corresponds to the conventions of the host
environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method returns a String value. The contents of the String are implementation-defined, but are
intended to represent the Date in the current time zone in a convenient, human-readable form
that corresponds to the conventions of the host environment's current
locale.
The meaning of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method returns a String value. The contents of the String are implementation-defined, but are
intended to represent the “time” portion of the Date in the current time zone in a
convenient, human-readable form that corresponds to the conventions of the host
environment's current locale.
The meaning of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
21.4.4.41 Date.prototype.toString ( )
This method performs the following steps when called:
For any Date d such that
d.[[DateValue]] is evenly divisible by 1000,
the result of Date.parse(d.toString()) = d.valueOf(). See
21.4.3.2.
Note 2
This method is not generic; it throws a TypeError exception if its
this value is not a Date. Therefore, it cannot be transferred to
other kinds of objects for use as a method.
21.4.4.41.1 TimeString ( tv )
The abstract operation TimeString takes argument tv (a Number, but not
NaN) and returns a String. It performs the following steps when
called:
Return the string-concatenation of
weekday, the code unit 0x0020 (SPACE), month, the code
unit 0x0020 (SPACE), day, the code unit 0x0020 (SPACE),
yearSign, and paddedYear.
Table 65: Names of days of the week
Number
Name
+0𝔽
"Sun"
1𝔽
"Mon"
2𝔽
"Tue"
3𝔽
"Wed"
4𝔽
"Thu"
5𝔽
"Fri"
6𝔽
"Sat"
Table 66: Names of months of the year
Number
Name
+0𝔽
"Jan"
1𝔽
"Feb"
2𝔽
"Mar"
3𝔽
"Apr"
4𝔽
"May"
5𝔽
"Jun"
6𝔽
"Jul"
7𝔽
"Aug"
8𝔽
"Sep"
9𝔽
"Oct"
10𝔽
"Nov"
11𝔽
"Dec"
21.4.4.41.3 TimeZoneString ( tv )
The abstract operation TimeZoneString takes argument tv (an integral
Number) and returns a String. It performs the following steps
when called:
Let tzName be an implementation-defined
string that is either the empty String or the string-concatenation of the
code unit 0x0020 (SPACE), the code unit 0x0028 (LEFT PARENTHESIS), an implementation-defined
timezone name, and the code unit 0x0029 (RIGHT PARENTHESIS).
Return the string-concatenation of
offsetSign, offsetHour, offsetMin, and
tzName.
21.4.4.41.4 ToDateString ( tv )
The abstract operation ToDateString takes argument tv (an integral
Number or NaN) and returns a String. It
performs the following steps when called:
This method returns a String value representing the instant in time corresponding to the
this value. The format of the String is based upon "HTTP-date" from RFC
7231, generalized to support the full range of times supported by ECMAScript Dates.
Return the string-concatenation of
weekday, ",", the code unit 0x0020 (SPACE),
day, the code unit 0x0020 (SPACE), month, the code unit 0x0020
(SPACE), yearSign, paddedYear, the code unit 0x0020 (SPACE),
and TimeString(tv).
21.4.4.44 Date.prototype.valueOf ( )
This method performs the following steps when called:
21.4.4.45 Date.prototype [ %Symbol.toPrimitive% ] (
hint )
This method is called by ECMAScript language operators to convert a Date to a primitive
value. The allowed values for hint are "default",
"number", and "string". Dates are unique among
built-in ECMAScript object in that they treat "default" as being
equivalent to "string", All other built-in ECMAScript objects treat
"default" as being equivalent to "number".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
The value of the "name" property of this method is
"[Symbol.toPrimitive]".
21.4.5 Properties of Date Instances
Date instances are ordinary objects that inherit properties from the
Date prototype
object. Date instances also have a [[DateValue]]
internal slot. The [[DateValue]] internal slot is the time value represented by this
Date.
is the initial value of the "String" property of the global
object.
creates and initializes a new String object when called as a constructor.
performs a type conversion when called as a function rather than as a constructor.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
String behaviour must include a super call to the String constructor to
create and initialize the subclass instance with a [[StringData]]
internal slot.
22.1.1.1 String ( value )
This function performs the following steps when called:
This function may be called with a variable number of arguments. The first argument is
template and the remainder of the arguments form the Listsubstitutions.
It performs the following steps when called:
Let substitutionCount be the number of elements in
substitutions.
This function is intended for use as a tag function of a Tagged Template (13.3.11). When called as such,
the first argument will be a well formed template object and the rest parameter will
contain the substitution values.
Unless explicitly stated otherwise, the methods of the String prototype object defined below are
not generic and the this value passed to them must be either a String value
or an object that has a [[StringData]] internal slot that has been
initialized to a String value.
This method returns a single element String containing the code unit at index
pos within the String value resulting from converting this object to a
String. If there is no element at that index, the result is the empty String. The
result is a
String value, not a String object.
If pos is an integral Number, then the result of
x.charAt(pos) is equivalent to the result of
x.substring(pos, pos + 1).
This method performs the following steps when called:
If position < 0 or position ≥ size, return the
empty String.
Return the substring of S from
position to position + 1.
Note 2
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.3 String.prototype.charCodeAt ( pos )
Note 1
This method returns a Number (a non-negative integral Number less
than 216) that is the numeric value of the code unit at index
pos within the String resulting from converting this object to a String.
If there is no element at that index, the result is NaN.
This method performs the following steps when called:
Return the Number value for the numeric value
of the code unit at index position within the String S.
Note 2
This method is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to
other kinds of objects for use as a method.
22.1.3.4 String.prototype.codePointAt ( pos )
Note 1
This method returns a non-negative integral Number less
than or equal to 0x10FFFF𝔽 that is the numeric value
of the UTF-16 encoded code point (6.1.4)
starting at the string element at index pos within the String resulting
from converting this object to a String. If there is no element at that index, the
result is undefined. If a valid UTF-16 surrogate
pair does not begin at pos, the result is the code
unit at pos.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to
other kinds of objects for use as a method.
22.1.3.5 String.prototype.concat ( ...args )
Note 1
When this method is called it returns the String value consisting of the code units
of the this value (converted to a String) followed by the code
units of each of the arguments converted to a String. The result is a String
value, not a String object.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to
other kinds of objects for use as a method.
22.1.3.6 String.prototype.constructor
The initial value of String.prototype.constructor is %String%.
If endPosition is undefined, let pos be
len; else let pos be ? ToIntegerOrInfinity(endPosition).
Let end be the result of clampingpos between 0 and
len.
Let searchLength be the length of searchStr.
If searchLength = 0, return true.
Let start be end - searchLength.
If start < 0, return false.
Let substring be the substring of S from
start to end.
If substring is searchStr, return true.
Return false.
Note 1
This method returns true if the sequence of code units of
searchString converted to a String is the same as the corresponding code
units of this object (converted to a String) starting at endPosition -
length(this). Otherwise it returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to
allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.8 String.prototype.includes ( searchString [ ,
position ] )
This method performs the following steps when called:
If searchString appears as a substring of the
result of converting this object to a String, at one or more indices that are
greater than or equal to position, this function returns
true; otherwise, it returns false. If
position is undefined, 0 is assumed, so as to search
all of the String.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to
allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.9 String.prototype.indexOf ( searchString [ ,
position ] )
Note 1
If searchString appears as a substring of the
result of converting this object to a String, at one or more indices that are
greater than or equal to position, then the smallest such index is
returned; otherwise, -1𝔽 is returned. If
position is undefined,
+0𝔽 is assumed, so as to search all of the String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.10 String.prototype.isWellFormed ( )
This method performs the following steps when called:
22.1.3.11 String.prototype.lastIndexOf ( searchString
[ , position ] )
Note 1
If searchString appears as a substring of the
result of converting this object to a String at one or more indices that are smaller
than or equal to position, then the greatest such index is returned;
otherwise, -1𝔽 is returned. If position is
undefined, the length of the String value is assumed, so as to
search all of the String.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method returns a Number other than NaN representing the result of an
implementation-defined
locale-sensitive String comparison of the this value (converted to a
String S) with that (converted to a String thatValue). The
result is intended to correspond with a sort order of String values according to
conventions of the host environment's current locale, and will
be negative when S is ordered before thatValue, positive when
S is ordered after thatValue, and zero in all other cases
(representing no relative ordering between S and thatValue).
Before performing the comparisons, this method performs the following steps to prepare the
Strings:
The meaning of the optional second and third parameters to this method are defined in the
ECMA-402 specification; implementations that do not include ECMA-402 support must not assign
any other interpretation to those parameter positions.
The actual return values are implementation-defined to permit
encoding additional information in them, but this method, when considered as a method of two
arguments, is required to be a consistent comparator defining a total
ordering on the set of all Strings. This method is also required to recognize and honour
canonical equivalence according to the Unicode Standard, including returning
+0𝔽 when comparing distinguishable Strings that are
canonically equivalent.
Note 1
This method itself is not directly suitable as an argument to
Array.prototype.sort because the latter requires a function of two
arguments.
Note 2
This method may rely on whatever language- and/or locale-sensitive comparison
functionality is available to the ECMAScript environment from the host
environment, and is intended to compare according to the
conventions of the host environment's current locale.
However, regardless of comparison capabilities, this method must recognize and
honour canonical equivalence according to the Unicode Standard—for example, the
following comparisons must all return +0𝔽:
// Å ANGSTROM SIGN vs.// Å LATIN CAPITAL LETTER A + COMBINING RING ABOVE"\u212B".localeCompare("A\u030A")
// Ω OHM SIGN vs.// Ω GREEK CAPITAL LETTER OMEGA"\u2126".localeCompare("\u03A9")
// ṩ LATIN SMALL LETTER S WITH DOT BELOW AND DOT ABOVE vs.// ṩ LATIN SMALL LETTER S + COMBINING DOT ABOVE + COMBINING DOT BELOW"\u1E69".localeCompare("s\u0307\u0323")
// ḍ̇ LATIN SMALL LETTER D WITH DOT ABOVE + COMBINING DOT BELOW vs.// ḍ̇ LATIN SMALL LETTER D WITH DOT BELOW + COMBINING DOT ABOVE"\u1E0B\u0323".localeCompare("\u1E0D\u0307")
// 가 HANGUL CHOSEONG KIYEOK + HANGUL JUNGSEONG A vs.// 가 HANGUL SYLLABLE GA"\u1100\u1161".localeCompare("\uAC00")
It is recommended that this method should not honour Unicode compatibility
equivalents or compatibility decompositions as defined in the Unicode Standard,
chapter 3, section 3.7.
Note 3
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.13 String.prototype.match ( regexp )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.14 String.prototype.matchAll ( regexp )
This method performs a regular expression match of the String representing the
this value against regexp and returns an iterator that yields match results.
Each match result is an Array containing the matched portion of the String as the first
element, followed by the portions matched by any capturing groups. If the regular expression
never matches, the returned iterator does not yield any match
results.
This method is intentionally generic, it does not require that
its this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
Note 2
Similarly to String.prototype.split,
String.prototype.matchAll is designed to typically act without mutating its
inputs.
22.1.3.15 String.prototype.normalize ( [ form ] )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to
other kinds of objects for use as a method.
The abstract operation StringPad takes arguments S (a String),
maxLength (a non-negative integer), fillString (a String), and
placement (start or end) and
returns a String. It performs the following steps when called:
Let stringLength be the length of S.
If maxLength ≤ stringLength, return S.
If fillString is the empty String, return S.
Let fillLen be maxLength - stringLength.
Let truncatedStringFiller be the String value consisting of repeated
concatenations of fillString truncated to length fillLen.
If placement is start, return the string-concatenation of
truncatedStringFiller and S.
The argument maxLength will be clamped such that it can be no smaller
than the length of S.
Note 2
The argument fillString defaults to " " (the String
value consisting of the code unit 0x0020 SPACE).
22.1.3.17.3 ToZeroPaddedDecimalString ( n,
minLength )
The abstract operation ToZeroPaddedDecimalString takes arguments n (a
non-negative integer) and minLength (a non-negative
integer) and
returns a String. It performs the following steps when called:
Let S be the String representation of n, formatted as a
decimal number.
Return the String value that is made from n copies of S
appended together.
Note 1
This method creates the String value consisting of the code units of the
this value (converted to String) repeated count times.
Note 2
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
The abstract operation GetSubstitution takes arguments matched (a String),
str (a String), position (a non-negative integer), captures (a
List of either
Strings or undefined), namedCaptures (an Object or
undefined), and replacementTemplate (a String) and returns
either a normal completion
containing a String or a throw
completion. For the purposes of this abstract operation, a
decimal digit is a code unit in the inclusive interval from
0x0030 (DIGIT ZERO) to 0x0039 (DIGIT NINE). It performs the following steps when called:
Repeat, while templateRemainder is not the empty String,
NOTE: The following steps isolate
ref (a prefix of templateRemainder), determine
refReplacement (its replacement), and then append that
replacement to result.
If templateRemainder starts with "$$",
then
Let ref be "$$".
Let refReplacement be "$".
Else if templateRemainder starts with
"$`", then
Let ref be "$`".
Let refReplacement be the substring of
str from 0 to position.
Else if templateRemainder starts with
"$&", then
Let ref be "$&".
Let refReplacement be matched.
Else if templateRemainder starts with "$'"
(0x0024 (DOLLAR SIGN) followed by 0x0027 (APOSTROPHE)), then
Let ref be "$'".
Let matchLength be the length of matched.
Let tailPos be position +
matchLength.
Let refReplacement be the substring of
str from min(tailPos,
stringLength).
NOTE: tailPos can exceed stringLength only
if this abstract operation was invoked by a call to the
intrinsic %Symbol.replace%
method of %RegExp.prototype%
on an object whose "exec" property is not the
intrinsic %RegExp.prototype.exec%.
Else if templateRemainder starts with "$"
followed by 1 or more decimal digits, then
If templateRemainder starts with
"$" followed by 2 or more decimal digits, let
digitCount be 2. Otherwise, let digitCount
be 1.
Let digits be the substring of
templateRemainder from 1 to 1 +
digitCount.
Let captureLen be the number of elements in
captures.
If index > captureLen and
digitCount = 2, then
NOTE: When a two-digit replacement pattern specifies an
index exceeding the count of capturing groups, it is
treated as a one-digit replacement pattern followed by a
literal digit.
Set digitCount to 1.
Set digits to the substring of
digits from 0 to 1.
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.22 String.prototype.slice ( start,
end )
This method returns a substring of the result of converting this
object to a String, starting from index start and running to, but not including,
index end (or through the end of the String if end is
undefined). If start is negative, it is treated as sourceLength + start where
sourceLength is the length of the String. If end is negative, it is
treated as sourceLength + end where
sourceLength is the length of the String. The result is a String value,
not a String object.
This method is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to
other kinds of objects for use as a method.
This method returns an Array into which substrings of the result of converting this object to
a String have been stored. The substrings are determined by searching from left to right for
occurrences of separator; these occurrences are not part of any String in the
returned array, but serve to divide up the String value. The value of separator
may be a String of any length or it may be an object, such as a RegExp, that has a %Symbol.split% method.
The value of separator may be an empty String. In this case,
separator does not match the empty substring
at the beginning or end of the input String, nor does it match the empty
substring at the end of the previous separator match. If
separator is the empty String, the String is split up into individual
code unit elements; the length of the result array equals the length of the String,
and each substring contains one code unit.
If the this value is (or converts to) the empty String, the result
depends on whether separator can match the empty String. If it can, the
result array contains no elements. Otherwise, the result array contains one element,
which is the empty String.
If separator is undefined, then the result array
contains just one String, which is the this value (converted to a
String). If limit is not undefined, then the output
array is truncated so that it contains no more than limit elements.
Note 2
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.24 String.prototype.startsWith ( searchString
[ , position ] )
This method performs the following steps when called:
If position is undefined, let pos be 0;
else let pos be ? ToIntegerOrInfinity(position).
Let start be the result of clampingpos between 0 and
len.
Let searchLength be the length of searchStr.
If searchLength = 0, return true.
Let end be start + searchLength.
If end > len, return false.
Let substring be the substring of S from
start to end.
If substring is searchStr, return true.
Return false.
Note 1
This method returns true if the sequence of code units of
searchString converted to a String is the same as the corresponding code
units of this object (converted to a String) starting at index position.
Otherwise it returns false.
Note 2
Throwing an exception if the first argument is a RegExp is specified in order to
allow future editions to define extensions that allow such argument values.
Note 3
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.25 String.prototype.substring ( start,
end )
This method returns a substring of the result of converting this
object to a String, starting from index start and running to, but not including,
index end of the String (or through the end of the String if end is
undefined). The result is a String value,
not a String object.
If either argument is NaN or negative, it is replaced with zero; if either
argument is strictly greater than the length of the String, it is replaced with the length
of the String.
If start is strictly greater than end, they are swapped.
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as
described in 6.1.4.
It works exactly the same as toLowerCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host
environment's current locale. There will only be a difference in the
few cases (such as Turkish) where the rules for that language conflict with the regular
Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
Note
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used:
This method interprets a String value as a sequence of UTF-16 encoded code points, as
described in 6.1.4.
It works exactly the same as toUpperCase except that it is intended to yield a
locale-sensitive result corresponding with conventions of the host
environment's current locale. There will only be a difference in the
few cases (such as Turkish) where the rules for that language conflict with the regular
Unicode case mappings.
The meaning of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
Note
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.28 String.prototype.toLowerCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as
described in 6.1.4.
The result must be derived according to the locale-insensitive case mappings in the Unicode
Character Database (this explicitly includes not only the file UnicodeData.txt,
but also all locale-insensitive mappings in the file SpecialCasing.txt
that accompanies it).
Note 1
The case mapping of some code points may produce multiple code points. In this case
the result String may not be the same length as the source String. Because both
toUpperCase and toLowerCase have context-sensitive
behaviour, the methods are not symmetrical. In other words,
s.toUpperCase().toLowerCase() is not necessarily equal to
s.toLowerCase().
Note 2
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.29 String.prototype.toString ( )
This method performs the following steps when called:
For a String object, this method happens to return the same thing as the
valueOf method.
22.1.3.30 String.prototype.toUpperCase ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as
described in 6.1.4.
It behaves in exactly the same way as String.prototype.toLowerCase, except that
the String is mapped using the toUppercase algorithm of the Unicode Default Case Conversion.
Note
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.32.1 TrimString ( string, where
)
The abstract operation TrimString takes arguments string (an ECMAScript language value)
and where (start, end, or
start+end) and returns either a normal completion
containing a String or a throw
completion. It interprets string as a sequence of
UTF-16 encoded code points, as described in 6.1.4. It
performs the following steps when called:
Let T be the String value that is a copy of S with
both leading and trailing white space removed.
Return T.
The definition of white space is the union of WhiteSpace and LineTerminator. When determining
whether a Unicode code point is in Unicode general category “Space_Separator” (“Zs”),
code unit sequences are interpreted as UTF-16 encoded code point sequences as specified
in 6.1.4.
22.1.3.33 String.prototype.trimEnd ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as
described in 6.1.4.
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.34 String.prototype.trimStart ( )
This method interprets a String value as a sequence of UTF-16 encoded code points, as
described in 6.1.4.
This method is intentionally generic; it does not require that its
this value be a String object. Therefore, it can be transferred
to other kinds of objects for use as a method.
22.1.3.35 String.prototype.valueOf ( )
This method performs the following steps when called:
The value of the "name" property of this method is
"[Symbol.iterator]".
22.1.4 Properties of String Instances
String instances are String exotic objects and have the internal
methods specified for such objects. String instances inherit properties from the String prototype
object. String instances also have a [[StringData]] internal slot. The [[StringData]] internal slot is the String value represented by this
String object.
String instances have a "length" property, and a set of enumerable properties
with integer-indexed names.
22.1.4.1 length
The number of elements in the String value represented by this String object.
Once a String object is initialized, this property is unchanging. It has the attributes {
[[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
22.1.5 String Iterator Objects
A String Iterator is an object that represents a specific iteration over
some specific String instance object. There is not a named constructor for String Iterator
objects. Instead, String Iterator objects are created by calling certain methods of String
instance objects.
The initial value of the %Symbol.toStringTag% property is
the String value "String Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
22.2 RegExp (Regular Expression) Objects
A RegExp object contains a regular expression and the associated flags.
Note
The form and functionality of regular expressions is modelled after the regular expression
facility in the Perl 5 programming language.
22.2.1 Patterns
The RegExp constructor applies the following grammar to the
input pattern String. An error occurs if the grammar cannot interpret the String as an expansion
of Pattern.
The abstract operation CountLeftCapturingParensWithin takes argument node (a
Parse Node) and returns a non-negative
integer. It
returns the number of left-capturing parentheses in node. A left-capturing parenthesis is
any ( pattern character that is matched by the ( terminal of the
Atom::(GroupSpecifieroptDisjunction) production.
The abstract operation CountLeftCapturingParensBefore takes argument node (a
Parse Node) and returns a non-negative
integer. It
returns the number of left-capturing
parentheses within the enclosing pattern that occur to the left of
node.
22.2.1.4 Static Semantics: MightBothParticipate ( x,
y )
The abstract operation MightBothParticipate takes arguments x (a Parse
Node) and y (a Parse Node) and returns
a Boolean. It performs the following steps when called:
The abstract operation GroupSpecifiersThatMatch takes argument thisGroupName (a
GroupNameParse
Node) and returns a List of GroupSpecifierParse
Nodes. It performs the following steps when called:
The syntax-directed
operation RegExpIdentifierCodePoints takes no arguments and returns a
List of code points.
It is defined piecewise over the following productions:
The syntax-directed
operation RegExpIdentifierCodePoint takes no arguments and returns a
code point. It is defined piecewise over the following productions:
A regular expression pattern is converted into an Abstract Closure using the
process described below. An implementation is encouraged to use more efficient algorithms than
the ones listed below, as long as the results are the same. The Abstract
Closure is used as the value of a RegExp object's [[RegExpMatcher]] internal slot.
A Pattern is a BMP pattern if its
associated flags contain neither a u nor a v. Otherwise, it is a
Unicode pattern. A BMP pattern matches against a String interpreted as consisting of a sequence
of 16-bit values that are Unicode code points in the range of the Basic Multilingual Plane. A
Unicode pattern matches against a String interpreted as consisting of Unicode code points
encoded using UTF-16. In the context of describing the behaviour of a BMP pattern “character”
means a single 16-bit Unicode BMP code point. In the context of describing the behaviour of a
Unicode pattern “character” means a UTF-16 encoded code point (6.1.4). In either
context, “character value” means the numeric value of the corresponding non-encoded code point.
The syntax and semantics of Pattern
is defined as if the source text for the Pattern was a List of SourceCharacter values where
each SourceCharacter
corresponds to a Unicode code point. If a BMP pattern contains a non-BMP SourceCharacter the entire
pattern is encoded using UTF-16 and the individual code units of that encoding are used as the
elements of the List.
Note
For example, consider a pattern expressed in source text as the single non-BMP character
U+1D11E (MUSICAL SYMBOL G CLEF). Interpreted as a Unicode pattern, it would be a single
element (character) List consisting of
the single code point U+1D11E. However, interpreted as a BMP pattern, it is first UTF-16
encoded to produce a two element List consisting of
the code units 0xD834 and 0xDD1E.
Patterns are passed to the RegExp constructor as ECMAScript String values in
which non-BMP characters are UTF-16 encoded. For example, the single character MUSICAL
SYMBOL G CLEF pattern, expressed as a String value, is a String of
length 2 whose elements were the code units 0xD834 and 0xDD1E. So no further translation
of the string would be necessary to process it as a BMP pattern consisting of two
pattern characters. However, to process it as a Unicode pattern UTF16SurrogatePairToCodePoint
must be used in producing a List whose sole
element is a single pattern character, the code point U+1D11E.
An implementation may not actually perform such translations to or from UTF-16, but the
semantics of this specification requires that the result of pattern matching be as if
such translations were performed.
22.2.2.1 Notation
The descriptions below use the following internal data structures:
A CharSetElement is one of the two following entities:
If rer.[[UnicodeSets]] is
false, then a CharSetElement is a character in the sense of
the Pattern Semantics above.
If rer.[[UnicodeSets]] is
true, then a CharSetElement is a sequence whose elements are
characters in the sense of the Pattern Semantics above. This includes the empty
sequence, sequences of one character, and sequences of more than one character.
For convenience, when working with CharSetElements of this kind, an individual
character is treated interchangeably with a sequence of one character.
A CharSet is a
mathematical set of CharSetElements.
A CaptureRange is a Record { [[StartIndex]], [[EndIndex]] } that
represents the range of characters included in a capture, where [[StartIndex]] is an integer representing the start
index (inclusive) of the range within Input, and [[EndIndex]] is an integer representing the end index (exclusive) of
the range within Input. For any CaptureRange, these
indices must satisfy the invariant that [[StartIndex]] ≤ [[EndIndex]].
A MatchState is a Record { [[Input]], [[EndIndex]], [[Captures]] } where [[Input]] is a
List of characters
representing the String being matched, [[EndIndex]] is an
integer, and
[[Captures]] is a List of values,
one for each left-capturing
parenthesis in the pattern. MatchStates are used to
represent partial match states in the regular expression matching algorithms. The [[EndIndex]] is one plus the index of the last input character
matched so far by the pattern, while [[Captures]] holds the
results of capturing parentheses. The nth element of [[Captures]] is either a CaptureRange
representing the range of characters captured by the nth set of
capturing parentheses, or undefined if the nth
set of capturing parentheses hasn't been reached yet. Due to backtracking, many
MatchStates may be in use at any time
during the matching process.
A MatcherContinuation is an Abstract Closure that
takes one MatchState argument and returns either
a MatchState or
failure. The MatcherContinuation attempts
to match the remaining portion (specified by the closure's captured values) of the
pattern against Input, starting at the intermediate state given by its
MatchState argument. If the match
succeeds, the MatcherContinuation returns
the final MatchState that it reached; if the
match fails, the MatcherContinuation returns
failure.
A Matcher is an
Abstract Closure that takes two
arguments—a MatchState and a MatcherContinuation—and
returns either a MatchState or
failure. A Matcher attempts to match a middle
subpattern (specified by the closure's captured values) of the pattern against the
MatchState's [[Input]], starting at the intermediate state given by its
MatchState argument. The MatcherContinuation argument
should be a closure that matches the rest of the pattern. After matching the subpattern
of a pattern to obtain a new MatchState, the Matcher then calls MatcherContinuation on that
new MatchState to test if the rest of the
pattern can match as well. If it can, the Matcher returns the
MatchState returned by MatcherContinuation; if not,
the Matcher may try different choices at its
choice points, repeatedly calling MatcherContinuation until it
either succeeds or all possibilities have been exhausted.
22.2.2.1.1 RegExp Records
A RegExp Record is a Record value used
to store information about a RegExp that is needed during compilation and possibly
during matching.
Let cap be a List
of rer.[[CapturingGroupsCount]]undefined values, indexed 1 through rer.[[CapturingGroupsCount]].
Let x be the MatchState
{ [[Input]]: Input, [[EndIndex]]: index, [[Captures]]: cap }.
Return m(x, c).
Note
A Pattern compiles to an Abstract Closure value.
RegExpBuiltinExec can then
apply this procedure to a List of
characters and an offset within that List to
determine whether the pattern would match starting at exactly that offset within the
List, and, if
it does match, what the values of the capturing parentheses would be. The algorithms
in 22.2.2 are designed so that
compiling a pattern may throw a SyntaxError exception; on the
other hand, once the pattern is successfully compiled, applying the resulting
Abstract Closure to find a match
in a List of
characters cannot throw an exception (except for any implementation-defined
exceptions that can occur anywhere such as out-of-memory).
The | regular expression operator separates two alternatives. The
pattern first tries to match the left Alternative (followed by the sequel of
the regular expression); if it fails, it tries to match the right Disjunction (followed
by the sequel of the regular expression). If the left Alternative, the right Disjunction, and the
sequel all have choice points, all choices in the sequel are tried before moving on
to the next choice in the left Alternative. If choices in the left
Alternative are
exhausted, the right Disjunction is tried instead of the
left Alternative.
Any capturing parentheses inside a portion of the pattern skipped by |
produce undefined values instead of Strings. Thus, for example,
Consecutive Terms try to
simultaneously match consecutive portions of Input. When
direction is forward, if the left Alternative, the right
Term, and the sequel of
the regular expression all have choice points, all choices in the sequel are tried
before moving on to the next choice in the right Term, and all choices in the right Term are tried before moving
on to the next choice in the left Alternative. When direction
is backward, the evaluation order of Alternative and Term are reversed.
The abstract operation RepeatMatcher takes arguments m (a Matcher), min (a non-negative
integer),
max (a non-negative integer or +∞), greedy (a Boolean),
x (a MatchState), c (a MatcherContinuation),
parenIndex (a non-negative integer), and parenCount (a
non-negative integer) and returns either a MatchState or
failure. It performs the following steps when called:
If max = 0, return c(x).
Let d be a new MatcherContinuation
with parameters (y) that captures m, min,
max, greedy, x, c,
parenIndex, and parenCount and performs the following
steps when called:
If min = 0 and y.[[EndIndex]] = x.[[EndIndex]], return
failure.
If min = 0, let min2 be 0; otherwise let
min2 be min - 1.
If max = +∞, let max2 be +∞; otherwise let
max2 be max - 1.
Return RepeatMatcher(m,
min2, max2, greedy, y,
c, parenIndex, parenCount).
Let cap be a copy of x.[[Captures]].
For each integerk in
the inclusive interval from
parenIndex + 1 to parenIndex + parenCount, set
cap[k] to undefined.
Let Input be x.[[Input]].
Let e be x.[[EndIndex]].
Let xr be the MatchState { [[Input]]: Input, [[EndIndex]]: e, [[Captures]]: cap }.
If min ≠ 0, return m(xr, d).
If greedy is false, then
Let z be c(x).
If z is not failure, return
z.
Return m(xr, d).
Let z be m(xr, d).
If z is not failure, return z.
Return c(x).
Note 1
An Atom followed by a
Quantifier is
repeated the number of times specified by the Quantifier. A Quantifier can be
non-greedy, in which case the Atom pattern is repeated as few times as
possible while still matching the sequel, or it can be greedy, in which case the
Atom pattern is
repeated as many times as possible while still matching the sequel. The Atom pattern is repeated
rather than the input character sequence that it matches, so different
repetitions of the Atom can match different input
substrings.
Note 2
If the Atom and the
sequel of the regular expression all have choice points, the Atom is first matched as
many (or as few, if non-greedy) times as possible. All choices in the sequel are
tried before moving on to the next choice in the last repetition of Atom. All choices in the
last (nth) repetition of Atom are tried before moving on to the
next choice in the next-to-last (n - 1)st repetition of Atom; at which point it
may turn out that more or fewer repetitions of Atom are now possible; these are
exhausted (again, starting with either as few or as many as possible) before
moving on to the next choice in the (n - 1)st repetition of Atom and so on.
Compare
/a[a-z]{2,4}/.exec("abcdefghi")
which returns "abcde" with
/a[a-z]{2,4}?/.exec("abcdefghi")
which returns "abc".
Consider also
/(aa|aabaac|ba|b|c)*/.exec("aabaac")
which, by the choice point ordering above, returns the array
["aaba", "ba"]
and not any of:
["aabaac", "aabaac"]
["aabaac", "c"]
The above ordering of choice points can be used to write a regular expression
that calculates the greatest common divisor of two numbers (represented in unary
notation). The following example calculates the gcd of 10 and 15:
Step 4 of the
RepeatMatcher clears Atom's captures each time Atom is repeated. We can
see its behaviour in the regular expression
/(z)((a+)?(b+)?(c))*/.exec("zaacbbbcac")
which returns the array
["zaacbbbcac", "z", "ac", "a", undefined, "c"]
and not
["zaacbbbcac", "z", "ac", "a", "bbb", "c"]
because each iteration of the outermost * clears all captured
Strings contained in the quantified Atom, which in this case includes capture
Strings numbered 2, 3, 4, and 5.
Note 4
Step 2.b of the RepeatMatcher
states that once the minimum number of repetitions has been satisfied, any more
expansions of Atom
that match the empty character sequence are not considered for further
repetitions. This prevents the regular expression engine from falling into an
infinite loop on patterns such as:
/(a*)*/.exec("b")
or the slightly more complicated:
/(a*)b\1+/.exec("baaaac")
which returns the array
["b", ""]
22.2.2.3.2 EmptyMatcher ( )
The abstract operation EmptyMatcher takes no arguments and returns a Matcher. It performs the following steps
when called:
Return a new Matcher with parameters
(x, c) that captures nothing and performs the following
steps when called:
The abstract operation MatchTwoAlternatives takes arguments m1 (a Matcher) and m2 (a Matcher) and returns a Matcher. It performs the following steps
when called:
Return a new Matcher with parameters
(x, c) that captures m1 and m2 and
performs the following steps when called:
The abstract operation MatchSequence takes arguments m1 (a Matcher), m2 (a Matcher), and direction
(forward or backward) and returns a
Matcher. It performs the following steps
when called:
If direction is forward, then
Return a new Matcher with parameters
(x, c) that captures m1 and
m2 and performs the following steps when called:
If e = 0, or if rer.[[Multiline]] is true and the
character Input[e - 1] is matched by LineTerminator, then
Return c(x).
Return failure.
Note 2
Even when the y flag is used with a pattern, ^ always
matches only at the beginning of Input, or (if rer.[[Multiline]] is true) at the beginning
of a line.
Let z be the MatchState
{ [[Input]]: Input, [[EndIndex]]: xe, [[Captures]]: cap }.
Return c(z).
Note 3
The form (?=Disjunction) specifies a
zero-width positive lookahead. In order for it to succeed, the pattern inside
Disjunction must
match at the current position, but the current position is not advanced before
matching the sequel. If Disjunction can match at the current
position in several ways, only the first one is tried. Unlike other regular
expression operators, there is no backtracking into a (?= form (this
unusual behaviour is inherited from Perl). This only matters when the Disjunction contains
capturing parentheses and the sequel of the pattern contains backreferences to those
captures.
For example,
/(?=(a+))/.exec("baaabac")
matches the empty String immediately after the first b and therefore
returns the array:
["", "aaa"]
To illustrate the lack of backtracking into the lookahead, consider:
The form (?!Disjunction) specifies a
zero-width negative lookahead. In order for it to succeed, the pattern inside
Disjunction must
fail to match at the current position. The current position is not advanced before
matching the sequel. Disjunction can contain capturing
parentheses, but backreferences to them only make sense from within Disjunction itself.
Backreferences to these capturing parentheses from elsewhere in the pattern always
return undefined because the negative lookahead must fail for the
pattern to succeed. For example,
/(.*?)a(?!(a+)b\2c)\2(.*)/.exec("baaabaac")
looks for an a not immediately followed by some positive number n of
a's, a b, another n a's (specified by the
first \2) and a c. The second \2 is outside
the negative lookahead, so it matches against undefined and
therefore always succeeds. The whole expression returns the array:
The abstract operation IsWordChar takes arguments rer (a RegExp
Record), Input (a List of
characters), and e (an integer) and returns a Boolean. It performs the
following steps when called:
Let InputLength be the number of elements in Input.
The syntax-directed
operation CompileQuantifier takes no arguments and returns a
Record with fields
[[Min]] (a non-negative integer), [[Max]] (a non-negative integer or +∞), and [[Greedy]] (a Boolean). It is defined piecewise over the following
productions:
The syntax-directed
operation CompileQuantifierPrefix takes no arguments and returns a
Record with fields
[[Min]] (a non-negative integer) and [[Max]] (a non-negative integer or +∞). It is defined piecewise over the
following productions:
If rer.[[UnicodeSets]] is
false, or if every CharSetElement of
cs consists of a single character (including if cs is empty),
return CharacterSetMatcher(rer,
cs, cc.[[Invert]],
direction).
Let r be the CaptureRange
{ [[StartIndex]]: ye,
[[EndIndex]]: xe }.
Set cap[parenIndex + 1] to r.
Let z be the MatchState { [[Input]]: Input, [[EndIndex]]: ye, [[Captures]]: cap }.
Return c(z).
Return m(x, d).
Note 2
Parentheses of the form (Disjunction) serve both
to group the components of the Disjunction pattern together and to
save the result of the match. The result can be used either in a backreference
(\ followed by a non-zero decimal number), referenced in a replace
String, or returned as part of an array from the regular expression matching
Abstract Closure. To inhibit the
capturing behaviour of parentheses, use the form (?:Disjunction) instead.
An escape sequence of the form \ followed by a non-zero decimal number
n matches the result of the nth set of capturing
parentheses (22.2.2.1). It is an error if the
regular expression has fewer than n capturing parentheses. If the regular
expression has n or more capturing parentheses but the
nth one is undefined because it has not
captured anything, then the backreference always succeeds.
If rer.[[UnicodeSets]] is
false, or if every CharSetElement of
cs consists of a single character (including if cs is empty),
return CharacterSetMatcher(rer,
cs, false, direction).
22.2.2.7.1 CharacterSetMatcher ( rer,
A, invert, direction )
The abstract operation CharacterSetMatcher takes arguments rer (a RegExp
Record), A (a CharSet),
invert (a Boolean), and direction (forward
or backward) and returns a Matcher. It performs the
following steps when called:
If there exists a CharSetElement in
A containing exactly one character a such that
Canonicalize(rer,
a) is cc, let found be
true. Otherwise, let found be
false.
If invert is false and found is
false, return failure.
If invert is true and found is
true, return failure.
Let cap be x.[[Captures]].
Let y be the MatchState { [[Input]]: Input, [[EndIndex]]: f, [[Captures]]: cap }.
Return c(y).
22.2.2.7.2 BackreferenceMatcher ( rer,
ns, direction )
The abstract operation BackreferenceMatcher takes arguments rer (a RegExp
Record), ns (a List of positive
integers),
and direction (forward or
backward) and returns a Matcher. It performs the
following steps when called:
Return a new Matcher with parameters
(x, c) that captures rer, ns, and
direction and performs the following steps when called:
If there exists an integeri in the
interval from 0 (inclusive) to
len (exclusive) such that Canonicalize(rer,
Input[rs + i]) is not Canonicalize(rer,
Input[g + i]), return
failure.
Let y be the MatchState { [[Input]]: Input, [[EndIndex]]: f, [[Captures]]: cap }.
Return c(y).
22.2.2.7.3 Canonicalize ( rer, ch )
The abstract operation Canonicalize takes arguments rer (a RegExp
Record) and ch (a character) and returns a character.
It performs the following steps when called:
If the file CaseFolding.txt
of the Unicode Character Database provides a simple or common case
folding mapping for ch, return the result of applying that
mapping to ch.
If the numeric value of ch ≥ 128 and the numeric value of
cu < 128, return ch.
Return cu.
Note
In case-insignificant matches when HasEitherUnicodeFlag(rer)
is true, all characters are implicitly case-folded using the
simple mapping provided by the Unicode Standard immediately before they are
compared. The simple mapping always maps to a single code point, so it does not
map, for example, ß (U+00DF LATIN SMALL LETTER SHARP S) to
ss or SS. It may however map code points outside the
Basic Latin block to code points within it—for example, ſ (U+017F
LATIN SMALL LETTER LONG S) case-folds to s (U+0073 LATIN SMALL
LETTER S) and K (U+212A KELVIN SIGN) case-folds to k
(U+006B LATIN SMALL LETTER K). Strings containing those code points are matched
by regular expressions such as /[a-z]/ui.
In case-insignificant matches when HasEitherUnicodeFlag(rer)
is false, the mapping is based on Unicode Default Case
Conversion algorithm toUppercase rather than toCasefold, which results in some
subtle differences. For example, Ω (U+2126 OHM SIGN) is mapped by
toUppercase to itself but by toCasefold to ω (U+03C9 GREEK SMALL
LETTER OMEGA) along with Ω (U+03A9 GREEK CAPITAL LETTER OMEGA), so
"\u2126" is matched by /[ω]/ui and
/[\u03A9]/ui but not by /[ω]/i or
/[\u03A9]/i. Also, no code point outside the Basic Latin block is
mapped to a code point within it, so strings such as "\u017F
ſ" and "\u212A K" are not matched by
/[a-z]/i.
22.2.2.7.4 UpdateModifiers ( rer, add,
remove )
The abstract operation UpdateModifiers takes arguments rer (a RegExp
Record), add (a String), and remove (a
String) and returns a RegExp Record. It performs the
following steps when called:
Assert: add and
remove have no elements in common.
Let ignoreCase be rer.[[IgnoreCase]].
Let multiline be rer.[[Multiline]].
Let dotAll be rer.[[DotAll]].
Let unicode be rer.[[Unicode]].
Let unicodeSets be rer.[[UnicodeSets]].
Let capturingGroupsCount be rer.[[CapturingGroupsCount]].
If remove contains "i", set ignoreCase
to false.
Else if add contains "i", set
ignoreCase to true.
If remove contains "m", set multiline
to false.
Else if add contains "m", set multiline
to true.
If remove contains "s", set dotAll to
false.
Else if add contains "s", set dotAll to
true.
Return the RegExp Record { [[IgnoreCase]]: ignoreCase, [[Multiline]]: multiline, [[DotAll]]: dotAll, [[Unicode]]: unicode, [[UnicodeSets]]: unicodeSets, [[CapturingGroupsCount]]:
capturingGroupsCount }.
22.2.2.8 Runtime Semantics: CompileCharacterClass
The syntax-directed
operation CompileCharacterClass takes argument rer (a
RegExp
Record) and returns a Record with fields
[[CharSet]] (a CharSet) and [[Invert]] (a Boolean). It is defined piecewise over the following
productions:
ClassContents can
expand into a single ClassAtom and/or ranges of two ClassAtom separated by
dashes. In the latter case the ClassContents includes all
characters between the first ClassAtom and the second ClassAtom, inclusive; an
error occurs if either ClassAtom does not represent a single
character (for example, if one is \w) or if the first ClassAtom's character value is strictly
greater than the second ClassAtom's character value.
Note 3
Even if the pattern ignores case, the case of the two ends of a range is significant
in determining which characters belong to the range. Thus, for example, the pattern
/[E-F]/i matches only the letters E, F,
e, and f, while the pattern /[E-f]/i matches
all uppercase and lowercase letters in the Unicode Basic Latin block as well as the
symbols [, \, ], ^,
_, and `.
Note 4
A - character can be treated literally or it can denote a range. It is
treated literally if it is the first or last character of ClassContents, the beginning or end
limit of a range specification, or immediately follows a range specification.
Let c be the character whose character value is cv.
Return the CharSet containing the single
character c.
Note 5
A ClassAtom can use
any of the escape sequences that are allowed in the rest of the regular expression
except for \b, \B, and backreferences. Inside a CharacterClass,
\b means the backspace character, while \B and
backreferences raise errors. Using a backreference inside a ClassAtom causes an
error.
Assert:
p is a binary Unicode property or binary property alias listed in the
“Property name and aliases” column of Table 70, or a binary
Unicode property of strings listed in the “Property name”
column of Table
71.
Let A be the CharSet containing all
CharSetElements whose character database definition includes the property
p with value “True”.
The result will often consist of two or more ranges. When UnicodeSets is
true and IgnoreCase is true, then MaybeSimpleCaseFolding(rer,
[Ā-č]) will include only the odd-numbered code points of that range.
Return the CharSet containing the single
character U+0008 (BACKSPACE).
22.2.2.9.1 CharacterRange ( A, B )
The abstract operation CharacterRange takes arguments A (a CharSet) and B (a CharSet) and returns a CharSet. It performs the following steps
when called:
Assert: A and B each
contain exactly one character.
Return the CharSet containing all characters
with a character value in the inclusive
interval from i to j.
22.2.2.9.2 HasEitherUnicodeFlag ( rer )
The abstract operation HasEitherUnicodeFlag takes argument rer (a RegExp
Record) and returns a Boolean. It performs the following steps
when called:
If rer.[[Unicode]] is
true or rer.[[UnicodeSets]] is true, then
Return true.
Return false.
22.2.2.9.3 WordCharacters ( rer )
The abstract operation WordCharacters takes argument rer (a RegExp
Record) and returns a CharSet. Returns a
CharSet containing the characters
considered "word characters" for the purposes of \b, \B,
\w, and \W It performs the following steps when called:
Let extraWordChars be the CharSet containing
all characters c such that c is not in
basicWordChars but Canonicalize(rer,
c) is in basicWordChars.
Assert: extraWordChars is empty
unless HasEitherUnicodeFlag(rer)
is true and rer.[[IgnoreCase]] is true.
Return the union of basicWordChars and extraWordChars.
22.2.2.9.4 AllCharacters ( rer )
The abstract operation AllCharacters takes argument rer (a RegExp
Record) and returns a CharSet. Returns the set
of “all characters” according to the regular expression flags. It performs the following
steps when called:
If rer.[[UnicodeSets]] is
true and rer.[[IgnoreCase]] is true, then
Return the CharSet
containing all Unicode code points c that do not have a Simple
Case Folding mapping (that is, scf(c)=c).
Return the CharSet containing all
code point values.
Else,
Return the CharSet containing all
code unit values.
22.2.2.9.5 MaybeSimpleCaseFolding ( rer,
A )
The abstract operation MaybeSimpleCaseFolding takes arguments rer (a RegExp
Record) and A (a CharSet) and returns a
CharSet. If rer.[[UnicodeSets]] is false or
rer.[[IgnoreCase]] is false, it
returns A. Otherwise, it uses the Simple Case
Folding (scf(cp)) definitions in the file CaseFolding.txt
of the Unicode Character Database (each of which maps a single code point to another
single code point) to map each CharSetElement of A
character-by-character into a canonical form and returns the resulting CharSet. It performs the following steps
when called:
If rer.[[UnicodeSets]] is
false or rer.[[IgnoreCase]] is false, return
A.
The abstract operation CharacterComplement takes arguments rer (a RegExp
Record) and S (a CharSet) and returns a
CharSet. It performs the following steps
when called:
Return the CharSet containing the
CharSetElements of A which are not also CharSetElements of
S.
22.2.2.9.7 UnicodeMatchProperty ( rer,
p )
The abstract operation UnicodeMatchProperty takes arguments rer (a RegExp
Record) and p (ECMAScript source text)
and returns a Unicode property name. It performs the following
steps when called:
If rer.[[UnicodeSets]] is
true and p is a Unicode property
name listed in the “Property name”
column of Table
71, then
Implementations must support the Unicode property names and aliases listed in Table 69, Table 70, and Table 71. To
ensure interoperability, implementations must not support any other property names or
aliases.
Note 1
For example, Script_Extensions (property name) and
scx (property alias) are valid, but script_extensions
or Scx aren't.
Note 2
The listed properties form a superset of what UTS18 RL1.2 requires.
Note 3
The spellings of entries in these tables (including casing) match the spellings
used in the file PropertyAliases.txt
in the Unicode Character Database. The precise spellings in that file are guaranteed
to be stable.
Table 69: Non-binary Unicode property aliases and their canonical
property names
The abstract operation UnicodeMatchPropertyValue takes arguments p (ECMAScript source
text) and v (ECMAScript source text)
and returns a Unicode property value. It performs the following steps when called:
Implementations must support the Unicode property values and property value aliases
listed in PropertyValueAliases.txt
for the properties listed in Table 69.
To ensure interoperability, implementations must not support any other property values
or property value aliases.
Note 1
For example, Xpeo and Old_Persian are valid
Script_Extensions values, but xpeo and
Old Persian aren't.
The syntax-directed
operation CompileClassSetString takes argument rer (a
RegExp
Record) and returns a sequence of characters. It is defined piecewise
over the following productions:
If F contains any code unit other than "d",
"g", "i", "m",
"s", "u", "v", or
"y", or if F contains any code unit more than once,
throw a SyntaxError exception.
If F contains "i", let i be
true; else let i be false.
If F contains "m", let m be
true; else let m be false.
If F contains "s", let s be
true; else let s be false.
If F contains "u", let u be
true; else let u be false.
If F contains "v", let v be
true; else let v be false.
Let rer be the RegExp Record { [[IgnoreCase]]: i, [[Multiline]]: m, [[DotAll]]: s, [[Unicode]]: u, [[UnicodeSets]]: v, [[CapturingGroupsCount]]:
capturingGroupsCount }.
Set obj.[[RegExpRecord]] to rer.
Set obj.[[RegExpMatcher]] to CompilePattern of
parseResult with argument rer.
22.2.3.4 Static Semantics: ParsePattern (
patternText, u, v )
The abstract operation ParsePattern takes arguments patternText (a sequence of
Unicode code points), u (a Boolean), and v (a Boolean) and returns a
Parse Node or a non-empty List of
SyntaxError objects.
is the initial value of the "RegExp" property of the global
object.
creates and initializes a new RegExp object when called as a constructor.
when called as a function rather than as a constructor, returns either a new RegExp object,
or the argument itself if the only argument is a RegExp object.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
RegExp behaviour must include a super call to the RegExp constructor to
create and initialize subclass instances with the necessary internal slots.
22.2.4.1 RegExp ( pattern, flags )
This function performs the following steps when called:
If pattern is supplied using a StringLiteral, the usual escape
sequence substitutions are performed before the String is processed by this
function. If pattern must contain an escape sequence to be recognized by this
function, any U+005C (REVERSE SOLIDUS) code points must be escaped within the
StringLiteral to
prevent them being removed when the contents of the StringLiteral are formed.
This function returns a copy of S in which characters that are potentially special
in a regular expression Pattern
have been replaced by equivalent escape sequences.
If escaped is the empty String and cp is matched by
either DecimalDigit or AsciiLetter,
then
NOTE: Escaping a leading digit ensures that output corresponds with
pattern text which may be used after a \0 character
escape or a DecimalEscape such
as \1 and still match S rather than be
interpreted as an extension of the preceding escape sequence.
Escaping a leading ASCII letter does the same for the context after
\c.
Despite having similar names, EscapeRegExpPattern and
RegExp.escape do not perform similar actions. The former escapes a
pattern for representation as a string, while this function escapes a string for
representation inside a pattern.
22.2.5.1.1 EncodeForRegExpEscape ( cp )
The abstract operation EncodeForRegExpEscape takes argument cp (a code point)
and returns a String. It returns a String representing a Pattern for matching cp. If
cp is white space or an ASCII punctuator, the returned value is an escape
sequence. Otherwise, the returned value is a String
representation of cp itself. It performs the following steps when called:
If cp is matched by SyntaxCharacter or
cp is U+002F (SOLIDUS), then
Else if cp is a code point listed in the “Code Point” column of
Table 67,
then
Return the string-concatenation
of 0x005C (REVERSE SOLIDUS) and the string in the “ControlEscape” column
of the row whose “Code Point” column contains cp.
Let otherPunctuators be the string-concatenation of
",-=<>#&!%:;@~'`" and the code unit 0x0022
(QUOTATION MARK).
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
22.2.5.3 get RegExp [ %Symbol.species% ]
RegExp[%Symbol.species%] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
RegExp prototype methods normally use their this value's constructor to create a derived object.
However, a subclass constructor may over-ride that default
behaviour by redefining its %Symbol.species% property.
The RegExp prototype object does not have a "valueOf" property of its
own; however, it inherits the "valueOf" property from the Object prototype
object.
22.2.6.1 RegExp.prototype.constructor
The initial value of RegExp.prototype.constructor is %RegExp%.
22.2.6.2 RegExp.prototype.exec ( string )
This method searches string for an occurrence of the regular expression pattern
and returns an Array containing the results of the match, or null if
string did not match.
RegExp.prototype.dotAll is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x0073 (LATIN SMALL LETTER S).
RegExp.prototype.flags is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
RegExp.prototype.global is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x0067 (LATIN SMALL LETTER G).
RegExp.prototype.hasIndices is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x0064 (LATIN SMALL LETTER D).
RegExp.prototype.ignoreCase is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x0069 (LATIN SMALL LETTER I).
The value of the "name" property of this method is
"[Symbol.match]".
Note
The %Symbol.match% property is
used by the IsRegExp abstract operation to identify
objects that have the basic behaviour of regular expressions. The absence of a
%Symbol.match% property or the
existence of such a property whose value does not Boolean coerce to
true indicates that the object is not intended to be used as a
regular expression object.
The value of the "name" property of this method is
"[Symbol.matchAll]".
22.2.6.10 get RegExp.prototype.multiline
RegExp.prototype.multiline is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x006D (LATIN SMALL LETTER M).
NOTE: When n = 1, the preceding step puts the first
element into captures (at index 0). More generally, the
nth capture (the characters captured by the
nth set of capturing parentheses) is at
captures[n - 1].
Let replacementString be ? GetSubstitution(matched,
S, position, captures,
namedCaptures, replaceValue).
If position ≥ nextSourcePosition, then
NOTE: position should not normally move backwards. If it
does, it is an indication of an ill-behaving RegExp subclass or use
of an access triggered side-effect to change the global flag or
other characteristics of rx. In such cases, the
corresponding substitution is ignored.
Set accumulatedResult to the string-concatenation
of accumulatedResult, the substring of
S from nextSourcePosition to
position, and replacementString.
Set nextSourcePosition to position +
matchLength.
If nextSourcePosition ≥ lengthS, return
accumulatedResult.
The value of the "name" property of this method is
"[Symbol.search]".
Note
The "lastIndex" and "global" properties of this
RegExp object are ignored when performing the search. The
"lastIndex" property is left unchanged.
22.2.6.13 get RegExp.prototype.source
RegExp.prototype.source is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
The abstract operation EscapeRegExpPattern takes arguments P (a String) and
F (a String) and returns a String. It performs the following steps when
called:
If F contains "v", then
Let patternSymbol be Pattern[+UnicodeMode,
+UnicodeSetsMode].
Else if F contains "u", then
Let patternSymbol be Pattern[+UnicodeMode,
~UnicodeSetsMode].
Else,
Let patternSymbol be Pattern[~UnicodeMode,
~UnicodeSetsMode].
Let S be a String in the form of a patternSymbol
equivalent to P interpreted as UTF-16 encoded Unicode code points
(6.1.4),
in which certain code points are escaped as described below. S may or
may not differ from P; however, the Abstract Closure that would
result from evaluating S as a patternSymbol must behave
identically to the Abstract Closure given by
the constructed object's [[RegExpMatcher]] internal
slot. Multiple calls to this abstract operation using the same values for
P and F must produce identical results.
The code points / or any LineTerminator occurring in the
pattern shall be escaped in S as necessary to ensure that the
string-concatenation of
"/", S, "/", and F
can be parsed (in an appropriate lexical context) as a RegularExpressionLiteral
that behaves identically to the constructed regular expression. For example, if
P is "/", then S could be
"\/" or "\u002F", among other
possibilities, but not "/", because /// followed
by F would be parsed as a SingleLineComment rather
than a RegularExpressionLiteral.
If P is the empty String, this specification can be met by letting
S be "(?:)".
Return S.
Note
Despite having similar names, RegExp.escape and EscapeRegExpPattern
do not perform similar actions. The former escapes a string for representation
inside a pattern, while this function escapes a pattern for representation as a
string.
This method returns an Array into which substrings of the result of converting
string to a String have been stored. The substrings are determined by
searching from left to right for matches of the this value
regular expression; these occurrences are not part of any String in the returned
array, but serve to divide up the String value.
The this value may be an empty regular expression or a regular
expression that can match an empty String. In this case, the regular expression does
not match the empty substring at the beginning or end of
the input String, nor does it match the empty substring
at the end of the previous separator match. (For example, if the regular expression
matches the empty String, the String is split up into individual code unit elements;
the length of the result array equals the length of the String, and each
substring contains one code unit.) Only the first match
at a given index of the String is considered, even if backtracking could yield a
non-empty substring match at that index. (For example,
/a*?/[Symbol.split]("ab") evaluates to the array
["a", "b"], while /a*/[Symbol.split]("ab") evaluates to
the array ["","b"].)
If string is (or converts to) the empty String, the result depends on
whether the regular expression can match the empty String. If it can, the result
array contains no elements. Otherwise, the result array contains one element, which
is the empty String.
If the regular expression contains capturing parentheses, then each time
separator is matched the results (including any
undefined results) of the capturing parentheses are spliced into
the output array. For example,
The value of the "name" property of this method is
"[Symbol.split]".
Note 2
This method ignores the value of the "global" and
"sticky" properties of this RegExp object.
22.2.6.15 get RegExp.prototype.sticky
RegExp.prototype.sticky is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x0079 (LATIN SMALL LETTER Y).
The returned String has the form of a RegularExpressionLiteral
that evaluates to another RegExp object with the same behaviour as this object.
22.2.6.18 get RegExp.prototype.unicode
RegExp.prototype.unicode is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x0075 (LATIN SMALL LETTER U).
RegExp.prototype.unicodeSets is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let R be the this value.
Let cu be the code unit 0x0076 (LATIN SMALL LETTER V).
The abstract operation RegExpExec takes arguments R (an Object) and S
(a String) and returns either a normal completion
containing either an Object or null, or a
throw completion. It
performs the following steps when called:
If a callable "exec" property is not found this algorithm falls
back to attempting to use the built-in RegExp matching algorithm. This provides
compatible behaviour for code written for prior editions where most built-in
algorithms that use regular expressions did not perform a dynamic property lookup of
"exec".
22.2.7.2 RegExpBuiltinExec ( R, S )
The abstract operation RegExpBuiltinExec takes arguments R (an initialized RegExp
instance) and S (a String) and returns either a normal completion
containing either an Array exotic object or
null, or a throw
completion. It performs the following steps when called:
Let length be the length of S.
Let lastIndex be ℝ(? ToLength(? Get(R,
"lastIndex"))).
Let flags be R.[[OriginalFlags]].
If flags contains "g", let global be
true; else let global be false.
If flags contains "y", let sticky be
true; else let sticky be false.
If flags contains "d", let hasIndices be
true; else let hasIndices be false.
If global is false and sticky is
false, set lastIndex to 0.
Let matcher be R.[[RegExpMatcher]].
If flags contains "u" or flags contains
"v", let fullUnicode be true; else
let fullUnicode be false.
Let matchSucceeded be false.
If fullUnicode is true, let input be
StringToCodePoints(S).
Otherwise, let input be a List whose
elements are the code units that are the elements of S.
NOTE: Each element of input is considered to be a character.
If capturedValue is not
undefined, append s to
matchedGroupNames.
NOTE: If there are multiple groups named s,
groups may already have an s property
at this point. However, because groups is an
ordinary
object whose properties are all
writable data
properties, the call to CreateDataPropertyOrThrow
is nevertheless guaranteed to succeed.
The abstract operation AdvanceStringIndex takes arguments S (a String),
index (a non-negative integer), and unicode (a Boolean) and
returns an integer. It performs the following steps when called:
The abstract operation GetStringIndex takes arguments S (a String) and
codePointIndex (a non-negative integer) and returns a non-negative integer. It interprets
S as a sequence of UTF-16 encoded code points, as described in 6.1.4, and returns
the code unit index corresponding to code point index codePointIndex when such an
index exists. Otherwise, it returns the length of S. It performs the following
steps when called:
If S is the empty String, return 0.
Let len be the length of S.
Let codeUnitCount be 0.
Let codePointCount be 0.
Repeat, while codeUnitCount < len,
If codePointCount = codePointIndex, return
codeUnitCount.
The number of code units from the start of a string at which the match
ends (exclusive).
22.2.7.6 GetMatchString ( S, match )
The abstract operation GetMatchString takes arguments S (a String) and
match (a Match Record) and returns a String. It
performs the following steps when called:
Assert:
match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
Return the substring of S from
match.[[StartIndex]] to match.[[EndIndex]].
22.2.7.7 GetMatchIndexPair ( S, match )
The abstract operation GetMatchIndexPair takes arguments S (a String) and
match (a Match Record) and returns an Array. It
performs the following steps when called:
Assert:
match.[[StartIndex]] ≤ match.[[EndIndex]] ≤ the length of S.
The abstract operation MakeMatchIndicesIndexPairArray takes arguments S (a
String), indices (a List of
either Match Records or
undefined), groupNames (a List of either Strings
or undefined), and hasGroups (a Boolean) and returns an Array.
It performs the following steps when called:
NOTE: If there are multiple groups named s,
groups may already have an s property
at this point. However, because groups is an
ordinary
object whose properties are all
writable data
properties, the call to CreateDataPropertyOrThrow
is nevertheless guaranteed to succeed.
RegExp instances are ordinary objects that inherit properties from the
RegExp prototype
object. RegExp instances have internal slots [[OriginalSource]], [[OriginalFlags]], [[RegExpRecord]], and [[RegExpMatcher]]. The
value of the [[RegExpMatcher]] internal slot is an Abstract
Closure representation of the Pattern of the RegExp object.
Note
Prior to ECMAScript 2015, RegExp instances were specified as having the own data
properties"source",
"global", "ignoreCase", and
"multiline". Those properties are now specified as accessor
properties of RegExp.prototype.
RegExp instances also have the following property:
22.2.8.1 lastIndex
The value of the "lastIndex" property specifies the String index at which
to start the next match. It is coerced to an integral Number when used (see
22.2.7.2). This property shall have the
attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
22.2.9 RegExp String Iterator Objects
A RegExp String Iterator is an object that represents a specific iteration
over some specific String instance object, matching against some specific RegExp instance
object. There is not a named constructor for RegExp String Iterator objects.
Instead, RegExp String Iterator objects are created by calling certain methods of RegExp
instance objects.
The abstract operation CreateRegExpStringIterator takes arguments R (an Object),
S (a String), global (a Boolean), and fullUnicode (a
Boolean) and returns an Object. It performs the following steps when called:
is the initial value of the "Array" property of the global
object.
creates and initializes a new Array when called as a constructor.
also creates and initializes a new Array when called as a function rather than as a
constructor. Thus the function call
Array(…) is equivalent to the object creation expression
new Array(…) with the same arguments.
is a function whose behaviour differs based upon the number and types of its arguments.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the exotic
Array behaviour must include a super call to the Array constructor to
initialize subclass instances that are Array exotic objects.
However, most of the Array.prototype methods are generic methods that are not
dependent upon their this value being an Array exotic
object.
23.1.1.1 Array ( ...values )
This function performs the following steps when called:
If NewTarget is undefined, let newTarget be the
active function object; else
let newTarget be NewTarget.
This method is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it
can be transferred to or inherited by any other constructors that may be
called with a single numeric argument.
23.1.2.2 Array.isArray ( arg )
This function performs the following steps when called:
This method is an intentionally generic factory method; it does not require that its
this value be the Array constructor. Therefore it
can be transferred to or inherited by other constructors that may be
called with a single numeric argument.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.1.2.5 get Array [ %Symbol.species% ]
Array[%Symbol.species%] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Array prototype methods normally use their this value's constructor to create a derived object.
However, a subclass constructor may over-ride that default
behaviour by redefining its %Symbol.species% property.
23.1.3 Properties of the Array Prototype Object
The Array prototype object:
is %Array.prototype%.
is an Array exotic object and has the internal
methods specified for such objects.
has a "length" property whose initial value is
+0𝔽 and whose attributes are { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
The Array prototype object is specified to be an Array exotic object to
ensure compatibility with ECMAScript code that was created prior to the ECMAScript 2015
specification.
The explicit setting of the "length" property in step 6 is intended to
ensure the length is correct when the final non-empty element of items
has trailing holes or when A is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
The initial value of Array.prototype.constructor is %Array%.
23.1.3.4 Array.prototype.copyWithin ( target,
start [ , end ] )
Note 1
The end argument is optional. If it is not provided, the length of the
this value is used.
Note 2
If target is negative, it is treated as length + target where
length is the length of the array. If start is negative, it is
treated as length + start.
If end is negative, it is treated as length + end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.5 Array.prototype.entries ( )
This method performs the following steps when called:
callback should be a function that accepts three arguments and returns a
value that is coercible to a Boolean value. every calls
callback once for each element present in the array, in ascending order,
until it finds one where callback returns false. If
such an element is found, every immediately returns
false. Otherwise, every returns
true. callback is called only for elements of the
array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the
this value for each invocation of callback. If it is
not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the
index of the element, and the object being traversed.
every does not directly mutate the object on which it is called but the
object may be mutated by the calls to callback.
The range of elements processed by every is set before the first call to
callback. Elements which are appended to the array after the call to
every begins will not be visited by callback. If existing
elements of the array are changed, their value as passed to callback will
be the value at the time every visits them; elements that are deleted
after the call to every begins and before being visited are not
visited. every acts like the "for all" quantifier in mathematics. In
particular, for an empty array, it returns true.
This method performs the following steps when called:
Let testResult be ToBoolean(? Call(callback,
thisArg, « kValue, 𝔽(k),
O »)).
If testResult is false, return
false.
Set k to k + 1.
Return true.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.7 Array.prototype.fill ( value [ ,
start [ , end ] ] )
Note 1
The start argument is optional. If it is not provided,
+0𝔽 is used.
The end argument is optional. If it is not provided, the length of the
this value is used.
Note 2
If start is negative, it is treated as length + start where
length is the length of the array. If end is negative, it is
treated as length + end.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
callback should be a function that accepts three arguments and returns a
value that is coercible to a Boolean value. filter calls
callback once for each element in the array, in ascending order, and
constructs a new array of all the values for which callback returns
true. callback is called only for elements of the
array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the
this value for each invocation of callback. If it is
not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the
index of the element, and the object being traversed.
filter does not directly mutate the object on which it is called but the
object may be mutated by the calls to callback.
The range of elements processed by filter is set before the first call
to callback. Elements which are appended to the array after the call to
filter begins will not be visited by callback. If existing
elements of the array are changed their value as passed to callback will
be the value at the time filter visits them; elements that are deleted
after the call to filter begins and before being visited are not
visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
This method calls predicate once for each element of the array, in
ascending index order, until it finds one where predicate returns a value
that coerces to true. If such an element is found,
find immediately returns that element value. Otherwise,
find returns undefined.
Let findRec be ? FindViaPredicate(O,
len, ascending, predicate,
thisArg).
Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
This method calls predicate once for each element of the array, in
ascending index order, until it finds one where predicate returns a value
that coerces to true. If such an element is found,
findIndex immediately returns the index of that element value.
Otherwise, findIndex returns -1.
Let findRec be ? FindViaPredicate(O,
len, ascending, predicate,
thisArg).
Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
This method calls predicate once for each element of the array, in
descending index order, until it finds one where predicate returns a
value that coerces to true. If such an element is found,
findLast immediately returns that element value. Otherwise,
findLast returns undefined.
Let findRec be ? FindViaPredicate(O,
len, descending, predicate,
thisArg).
Return findRec.[[Value]].
Note 2
This method is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to
other kinds of objects for use as a method.
This method calls predicate once for each element of the array, in
descending index order, until it finds one where predicate returns a
value that coerces to true. If such an element is found,
findLastIndex immediately returns the index of that element value.
Otherwise, findLastIndex returns -1.
Let findRec be ? FindViaPredicate(O,
len, descending, predicate,
thisArg).
Return findRec.[[Index]].
Note 2
This method is intentionally generic; it does not require that its
this value be an Array object. Therefore it can be transferred to
other kinds of objects for use as a method.
23.1.3.12.1 FindViaPredicate ( O, len,
direction, predicate, thisArg )
O should be an array-like object or a TypedArray.
This operation calls predicate once for each element of O, in
either ascending index order or descending index order (as indicated by
direction), until it finds one where predicate returns a value
that coerces to true. At that point, this operation returns a
Record that gives
the index and value of the element found. If no such element is found, this operation
returns a Record that
specifies -1𝔽 for the index and
undefined for the value.
predicate should be a function. When called for an element of the array, it is
passed three arguments: the value of the element, the index of the element, and the
object being traversed. Its return value will be coerced to a Boolean value.
thisArg will be used as the this value for each invocation
of predicate.
This operation does not directly mutate the object on which it is called, but the object
may be mutated by the calls to predicate.
The range of elements processed is set before the first call to predicate,
just before the traversal begins. Elements that are appended to the array after this
will not be visited by predicate. If existing elements of the array are
changed, their value as passed to predicate will be the value at the time
that this operation visits them. Elements that are deleted after traversal begins and
before being visited are still visited and are either looked up from the prototype or
are undefined.
It performs the following steps when called:
If IsCallable(predicate)
is false, throw a TypeError exception.
If direction is ascending, then
Let indices be a List
of the integers in the interval from 0 (inclusive) to
len (exclusive), in ascending order.
Else,
Let indices be a List
of the integers in the interval from 0 (inclusive) to
len (exclusive), in descending order.
callback should be a function that accepts three arguments.
forEach calls callback once for each element present in the
array, in ascending order. callback is called only for elements of the
array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the
this value for each invocation of callback. If it is
not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the
index of the element, and the object being traversed.
forEach does not directly mutate the object on which it is called but
the object may be mutated by the calls to callback.
The range of elements processed by forEach is set before the first call
to callback. Elements which are appended to the array after the call to
forEach begins will not be visited by callback. If existing
elements of the array are changed, their value as passed to callback will
be the value at the time forEach visits them; elements that are deleted
after the call to forEach begins and before being visited are not
visited.
This method performs the following steps when called:
Perform ? Call(callback,
thisArg, « kValue, 𝔽(k),
O »).
Set k to k + 1.
Return undefined.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
This method compares searchElement to the elements of the array, in
ascending order, using the SameValueZero algorithm, and if
found at any position, returns true; otherwise, it returns
false.
The optional second argument fromIndex defaults to
+0𝔽 (i.e. the whole array is searched). If it is
greater than or equal to the length of the array, false is
returned, i.e. the array will not be searched. If it is less than
-0𝔽, it is used as the offset from the end of the
array to compute fromIndex. If the computed index is less than or equal
to +0𝔽, the whole array will be searched.
This method performs the following steps when called:
If SameValueZero(searchElement,
elementK) is true, return
true.
Set k to k + 1.
Return false.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
Note 3
This method intentionally differs from the similar indexOf method in two
ways. First, it uses the SameValueZero algorithm, instead of
IsStrictlyEqual, allowing it to
detect NaN array elements. Second, it does not skip missing array
elements, instead treating them as undefined.
This method compares searchElement to the elements of the array, in ascending
order, using the IsStrictlyEqual algorithm, and if found
at one or more indices, returns the smallest such index; otherwise, it returns
-1𝔽.
Note 1
The optional second argument fromIndex defaults to
+0𝔽 (i.e. the whole array is searched). If it is
greater than or equal to the length of the array, -1𝔽
is returned, i.e. the array will not be searched. If it is less than
-0𝔽, it is used as the offset from the end of the
array to compute fromIndex. If the computed index is less than or equal
to +0𝔽, the whole array will be searched.
This method performs the following steps when called:
If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
Set k to k + 1.
Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.18 Array.prototype.join ( separator )
This method converts the elements of the array to Strings, and then concatenates these
Strings, separated by occurrences of the separator. If no separator is provided,
a single comma is used as the separator.
This method is intentionally generic; it does not require that its
this value be an Array. Therefore, it can be transferred to other
kinds of objects for use as a method.
23.1.3.19 Array.prototype.keys ( )
This method performs the following steps when called:
This method compares searchElement to the elements of the array in
descending order using the IsStrictlyEqual algorithm, and if
found at one or more indices, returns the largest such index; otherwise, it returns
-1𝔽.
The optional second argument fromIndex defaults to the array's length
minus one (i.e. the whole array is searched). If it is greater than or equal to the
length of the array, the whole array will be searched. If it is less than
-0𝔽, it is used as the offset from the end of the
array to compute fromIndex. If the computed index is less than or equal
to +0𝔽, -1𝔽 is
returned.
This method performs the following steps when called:
If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
Set k to k - 1.
Return -1𝔽.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
callback should be a function that accepts three arguments.
map calls callback once for each element in the array, in
ascending order, and constructs a new Array from the results. callback is
called only for elements of the array which actually exist; it is not called for
missing elements of the array.
If a thisArg parameter is provided, it will be used as the
this value for each invocation of callback. If it is
not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the
index of the element, and the object being traversed.
map does not directly mutate the object on which it is called but the
object may be mutated by the calls to callback.
The range of elements processed by map is set before the first call to
callback. Elements which are appended to the array after the call to
map begins will not be visited by callback. If existing
elements of the array are changed, their value as passed to callback will
be the value at the time map visits them; elements that are deleted
after the call to map begins and before being visited are not visited.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.22 Array.prototype.pop ( )
Note 1
This method removes the last element of the array and returns it.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.23 Array.prototype.push ( ...items )
Note 1
This method appends the arguments to the end of the array, in the order in which they
appear. It returns the new length of the array.
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
callback should be a function that takes four arguments.
reduce calls the callback, as a function, once for each element after
the first element present in the array, in ascending order.
callback is called with four arguments: the previousValue
(value from the previous call to callback), the currentValue
(value of the current element), the currentIndex, and the object being
traversed. The first time that callback is called, the previousValue and
currentValue can be one of two values. If an initialValue was
supplied in the call to reduce, then previousValue will be
initialValue and currentValue will be the first value in the
array. If no initialValue was supplied, then previousValue
will be the first value in the array and currentValue will be the second.
It is a TypeError if the array contains no elements and
initialValue is not provided.
reduce does not directly mutate the object on which it is called but the
object may be mutated by the calls to callback.
The range of elements processed by reduce is set before the first call
to callback. Elements that are appended to the array after the call to
reduce begins will not be visited by callback. If existing
elements of the array are changed, their value as passed to callback will
be the value at the time reduce visits them; elements that are deleted
after the call to reduce begins and before being visited are not
visited.
This method performs the following steps when called:
Set accumulator to ? Call(callback,
undefined, « accumulator,
kValue, 𝔽(k),
O »).
Set k to k + 1.
Return accumulator.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
callback should be a function that takes four arguments.
reduceRight calls the callback, as a function, once for each element
after the first element present in the array, in descending order.
callback is called with four arguments: the previousValue
(value from the previous call to callback), the currentValue
(value of the current element), the currentIndex, and the object being
traversed. The first time the function is called, the previousValue and
currentValue can be one of two values. If an initialValue was
supplied in the call to reduceRight, then previousValue will
be initialValue and currentValue will be the last value in the
array. If no initialValue was supplied, then previousValue
will be the last value in the array and currentValue will be the
second-to-last value. It is a TypeError if the array contains no
elements and initialValue is not provided.
reduceRight does not directly mutate the object on which it is called
but the object may be mutated by the calls to callback.
The range of elements processed by reduceRight is set before the first
call to callback. Elements that are appended to the array after the call
to reduceRight begins will not be visited by callback. If
existing elements of the array are changed by callback, their value as
passed to callback will be the value at the time reduceRight
visits them; elements that are deleted after the call to reduceRight
begins and before being visited are not visited.
This method performs the following steps when called:
Set accumulator to ? Call(callback,
undefined, « accumulator,
kValue, 𝔽(k),
O »).
Set k to k - 1.
Return accumulator.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.26 Array.prototype.reverse ( )
Note 1
This method rearranges the elements of the array so as to reverse their order. It
returns the reversed array.
This method performs the following steps when called:
Assert: lowerExists
and upperExists are both false.
NOTE: No action is required.
Set lower to lower + 1.
Return O.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore, it can be transferred to other
kinds of objects for use as a method.
23.1.3.27 Array.prototype.shift ( )
This method removes the first element of the array and returns it.
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.28 Array.prototype.slice ( start,
end )
This method returns an array containing the elements of the array from element
start up to, but not including, element end (or through the end of the
array if end is undefined). If start is negative,
it is treated as length + start
where length is the length of the array. If end is negative, it is
treated as length + end where
length is the length of the array.
The explicit setting of the "length" property in step 15 is intended to
ensure the length is correct even when A is not a built-in Array.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
callback should be a function that accepts three arguments and returns a
value that is coercible to a Boolean value. some calls
callback once for each element present in the array, in ascending order,
until it finds one where callback returns true. If
such an element is found, some immediately returns
true. Otherwise, some returns
false. callback is called only for elements of the
array which actually exist; it is not called for missing elements of the array.
If a thisArg parameter is provided, it will be used as the
this value for each invocation of callback. If it is
not provided, undefined is used instead.
callback is called with three arguments: the value of the element, the
index of the element, and the object being traversed.
some does not directly mutate the object on which it is called but the
object may be mutated by the calls to callback.
The range of elements processed by some is set before the first call to
callback. Elements that are appended to the array after the call to
some begins will not be visited by callback. If existing
elements of the array are changed, their value as passed to callback will
be the value at the time that some visits them; elements that are
deleted after the call to some begins and before being visited are not
visited. some acts like the "exists" quantifier in mathematics. In
particular, for an empty array, it returns false.
This method performs the following steps when called:
Let testResult be ToBoolean(? Call(callback,
thisArg, « kValue, 𝔽(k),
O »)).
If testResult is true, return
true.
Set k to k + 1.
Return false.
Note 2
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.30 Array.prototype.sort ( comparator )
This method sorts the elements of this array. If comparator is not
undefined, it should be a function that accepts two arguments
x and y and returns a negative Number if x <
y, a positive Number if x > y, or a zero otherwise.
It performs the following steps when called:
If comparator is not
undefined and IsCallable(comparator) is
false, throw a TypeError exception.
NOTE: The call to SortIndexedProperties in
step 5 uses
skip-holes. The remaining indices are deleted to preserve the
number of holes that were detected and excluded from the sort.
Because non-existent property values always compare greater than
undefined property values, and undefined
always compares greater than any other value (see CompareArrayElements),
undefined property values always sort to the end of the result,
followed by non-existent property values.
This method is intentionally generic; it does not require that its
this value be an Array. Therefore, it can be transferred to other
kinds of objects for use as a method.
There must be some mathematical permutation π of the non-negative integers less
than itemCount, such that for every non-negative integerj less than itemCount, the element old[j] is exactly the same as new[π(j)].
Then for all non-negative integersj and k, each
less than itemCount, if ℝ(SortCompare(old[j],
old[k])) < 0, then π(j) < π(k).
And for all non-negative integersj and k such
that j < k < itemCount, if ℝ(SortCompare(old[j],
old[k])) = 0, then π(j)
< π(k); i.e., the sort is stable.
Here the notation old[j] is used to refer to
items[j] before step 4 is
executed, and the notation new[j] to refer
to items[j] after step 4
has been executed.
An abstract closure or function comparator is a consistent comparator for a set of
values S if all of the requirements below are met for all values
a, b, and c (possibly the same value) in the set
S: The notation a <Cb means ℝ(comparator(a,
b)) < 0; a =Cb means ℝ(comparator(a,
b)) = 0; and a
>Cb means ℝ(comparator(a,
b)) > 0.
Calling comparator(a, b) always returns the same
value v when given a specific pair of values a and
b as its two arguments. Furthermore, vis a
Number, and v is not NaN. Note
that this implies that exactly one of a <Cb,
a =Cb, and a >Cb will be true for a given pair of a and b.
Calling comparator(a, b) does not modify
obj or any object on obj's prototype chain.
a =Ca (reflexivity)
If a =Cb, then b =Ca (symmetry)
If a =Cb and b =Cc, then a =Cc (transitivity of
=C)
If a <Cb and b <Cc, then a <Cc (transitivity of
<C)
If a >Cb and b >Cc, then a >Cc (transitivity of
>C)
Note
The above conditions are necessary and sufficient to ensure that
comparator divides the set S into equivalence classes and
that these equivalence classes are totally ordered.
23.1.3.30.2 CompareArrayElements ( x,
y, comparator )
This method deletes the deleteCount elements of the array starting at
integer
indexstart and replaces them with the elements of
items. It returns an Array containing the deleted elements (if any).
This method performs the following steps when called:
The explicit setting of the "length" property in steps 15 and 20 is intended
to ensure the lengths are correct even when the objects are not built-in Arrays.
Note 3
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
An ECMAScript implementation that includes the ECMA-402 Internationalization API must
implement this method as specified in the ECMA-402 specification. If an ECMAScript
implementation does not include the ECMA-402 API the following specification of this method
is used.
Note 1
The first edition of ECMA-402 did not include a replacement specification for this
method.
The meanings of the optional parameters to this method are defined in the ECMA-402
specification; implementations that do not include ECMA-402 support must not use those
parameter positions for anything else.
This method performs the following steps when called:
This method converts the elements of the array to Strings using their
toLocaleString methods, and then concatenates these Strings, separated
by occurrences of an implementation-defined
locale-sensitive separator String. This method is analogous to toString
except that it is intended to yield a locale-sensitive result corresponding with
conventions of the host environment's current locale.
Note 3
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.33 Array.prototype.toReversed ( )
This method performs the following steps when called:
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.37 Array.prototype.unshift ( ...items )
This method prepends the arguments to the start of the array, such that their order within
the array is the same as the order in which they appear in the argument list.
This method is intentionally generic; it does not require that its
this value be an Array. Therefore it can be transferred to other
kinds of objects for use as a method.
23.1.3.38 Array.prototype.values ( )
This method performs the following steps when called:
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
Note
The own property names of this object are property names that were not included as
standard properties of Array.prototype prior to the ECMAScript 2015
specification. These names are ignored for with statement binding
purposes in order to preserve the behaviour of existing code that might use one of
these names as a binding in an outer scope that is shadowed by a with
statement whose binding object is an Array.
The reason that "with" is not included in the
unscopableList is because it is already a reserved word.
Array instances have a "length" property, and a set of enumerable properties
with array
index names.
23.1.4.1 length
The "length" property of an Array instance is a data
property whose value is always numerically greater than the name of
every configurable own property whose name is an array index.
The "length" property initially has the attributes { [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: false }.
Note
Reducing the value of the "length" property has the side-effect of
deleting own array elements whose array index is between the old and new
length values. However, non-configurable properties can not be deleted. Attempting
to set the "length" property of an Array to a value that is
numerically less than or equal to the largest numeric own property
name of an existing non-configurable array-indexed property of the array will
result in the length being set to a numeric value that is one greater than that
non-configurable numeric own property name. See 10.4.2.1.
23.1.5 Array Iterator Objects
An Array Iterator is an object that represents a specific iteration over
some specific Array instance object. There is not a named constructor for Array Iterator
objects. Instead, Array Iterator objects are created by calling certain methods of Array
instance objects.
23.1.5.1 CreateArrayIterator ( array, kind
)
The abstract operation CreateArrayIterator takes arguments array (an Object) and
kind (key+value, key, or
value) and returns an Object. It is used to create iterator objects for Array methods
that return such iterators. It performs the following
steps when called:
A value that identifies what is returned for each element of the
iteration.
23.2 TypedArray Objects
A TypedArray presents an array-like view of an underlying binary data buffer (25.1). A TypedArray element type is the
underlying binary scalar data type that all elements of a TypedArray instance have. There
is a distinct TypedArrayconstructor, listed in Table 75, for each of the supported
element types. Each constructor in Table 75 has a corresponding
distinct prototype object.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.2.2.4 get %TypedArray% [ %Symbol.species% ]
%TypedArray%[%Symbol.species%]
is an accessor
property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
%TypedArray%.prototype.buffer
is an accessor
property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
Assert:
O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
23.2.3.3 get %TypedArray%.prototype.byteLength
%TypedArray%.prototype.byteLength
is an accessor
property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
%TypedArray%.prototype.byteOffset
is an accessor
property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
If IsStrictlyEqual(searchElement,
elementK) is true, return 𝔽(k).
Set k to k - 1.
Return -1𝔽.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
23.2.3.21 get %TypedArray%.prototype.length
%TypedArray%.prototype.length
is an accessor
property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
This method sets multiple values in this TypedArray, reading the values from
source. The details differ based upon the type of source. The optional
offset value indicates the first element index in this TypedArray
where values are written. If omitted, it is assumed to be 0.
The abstract operation SetTypedArrayFromArrayLike takes arguments target (a
TypedArray), targetOffset (a
non-negative integer or +∞), and source (an
ECMAScript language value,
but not a TypedArray) and returns either a normal completion
containingunused or a throw
completion. It sets multiple values in target,
starting at index targetOffset, reading the values from source. It
performs the following steps when called:
The abstract operation SetTypedArrayFromTypedArray takes arguments target (a
TypedArray), targetOffset (a
non-negative integer or +∞), and source (a
TypedArray) and returns either a normal completion
containingunused or a throw
completion. It sets multiple values in target,
starting at index targetOffset, reading the values from source. It
performs the following steps when called:
If targetOffset = +∞, throw a RangeError
exception.
If srcLength + targetOffset > targetLength,
throw a RangeError exception.
If target.[[ContentType]] is not
source.[[ContentType]], throw a
TypeError exception.
If IsSharedArrayBuffer(srcBuffer)
is true, IsSharedArrayBuffer(targetBuffer)
is true, and srcBuffer.[[ArrayBufferData]] is targetBuffer.[[ArrayBufferData]], let
sameSharedArrayBuffer be true; otherwise, let
sameSharedArrayBuffer be false.
If SameValue(srcBuffer,
targetBuffer) is true or
sameSharedArrayBuffer is true, then
This is a distinct method that, except as described below, implements the same requirements
as those of Array.prototype.sort as defined in 23.1.3.30. The implementation of
this method may be optimized with the knowledge that the this value is an
object that has a fixed length and whose integer-indexed properties are not sparse.
This method is not generic. The this value must be an object with a [[TypedArrayName]] internal slot.
It performs the following steps when called:
If comparator is not undefined and IsCallable(comparator) is
false, throw a TypeError exception.
Because NaN always compares greater than any other value (see
CompareTypedArrayElements),
NaN property values always sort to the end of the result when
comparator is not provided.
23.2.3.30 %TypedArray%.prototype.subarray ( start,
end )
This method returns a new TypedArray whose element type is the element type of
this TypedArray and whose ArrayBuffer is the ArrayBuffer of this
TypedArray, referencing the elements in the interval from start
(inclusive) to end (exclusive). If either start or end is
negative, it refers to an index from the end of the array, as opposed to from the beginning.
This is a distinct method that implements the same algorithm as
Array.prototype.toLocaleString as defined in 23.1.3.32 except that
TypedArrayLength is called in place of
performing a [[Get]] of "length". The
implementation of the algorithm may be optimized with the knowledge that the
this value has a fixed length when the underlying buffer is not resizable
and whose integer-indexed properties are not sparse.
However, such optimization must not introduce any observable changes in the specified
behaviour of the algorithm.
This method is not generic. ValidateTypedArray is called with the
this value and seq-cst as arguments prior to
evaluating the algorithm. If its result is an abrupt completion
that exception is thrown instead of evaluating the algorithm.
Note
If the ECMAScript implementation includes the ECMA-402 Internationalization API this
method is based upon the algorithm for Array.prototype.toLocaleString
that is in the ECMA-402 specification.
23.2.3.32 %TypedArray%.prototype.toReversed ( )
This method performs the following steps when called:
The initial value of the %Symbol.iterator% property is
%TypedArray.prototype.values%, defined in 23.2.3.35.
23.2.3.38 get %TypedArray%.prototype [ %Symbol.toStringTag% ]
%TypedArray%.prototype[%Symbol.toStringTag%]
is an accessor
property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
The abstract operation TypedArrayElementSize takes argument O (a TypedArray) and
returns a non-negative integer. It performs the following steps when called:
Return the Element Size value specified in Table 75 for
O.[[TypedArrayName]].
23.2.4.6 TypedArrayElementType ( O )
The abstract operation TypedArrayElementType takes argument O (a TypedArray) and
returns a TypedArray element type. It performs
the following steps when called:
Return the Element Type value specified in Table 75 for
O.[[TypedArrayName]].
23.2.4.7 CompareTypedArrayElements ( x, y,
comparator )
The abstract operation CompareTypedArrayElements takes arguments x (a Number or a
BigInt), y (a Number or a BigInt), and comparator (a function
object or undefined) and returns either a
normal completion
containing a Number or an abrupt completion.
It performs the following steps when called:
is an intrinsic object that has the structure described below, differing only in the name
used as the constructor name instead of
TypedArray, in Table 75.
is a function whose behaviour differs based upon the number and types of its arguments. The
actual behaviour of a call of TypedArray depends upon the number and kind of
arguments that are passed to it.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
TypedArray behaviour must include a super call to the
TypedArrayconstructor to create and initialize the subclass
instance with the internal state necessary to support the %TypedArray%.prototype
built-in methods.
23.2.5.1TypedArray ( ...args )
Each TypedArrayconstructor performs the following steps when
called:
If NewTarget is undefined, throw a TypeError
exception.
Let constructorName be the String value of the Constructor Name value specified in
Table 75 for this
TypedArrayconstructor.
Let proto be "%TypedArray.prototype%".
Let numberOfArgs be the number of elements in args.
The abstract operation AllocateTypedArray takes arguments constructorName (a
String which is the name of a TypedArrayconstructor in Table 75),
newTarget (a constructor), and defaultProto (a
String) and optional argument length (a non-negative integer) and
returns either a normal
completion containing a TypedArray or a throw
completion. It is used to validate and create an instance of a
TypedArrayconstructor. If the
length argument is passed, an ArrayBuffer of that length is also allocated
and associated with the new TypedArray instance. AllocateTypedArray
provides common semantics that is used by TypedArray. It performs the
following steps when called:
23.2.5.1.5 InitializeTypedArrayFromArrayLike ( O,
arrayLike )
The abstract operation InitializeTypedArrayFromArrayLike takes arguments O (a
TypedArray) and arrayLike (an
Object, but not a TypedArray or an ArrayBuffer) and returns
either a normal completion
containingunused or a throw
completion. It performs the following steps when called:
The abstract operation AllocateTypedArrayBuffer takes arguments O (a TypedArray)
and length (a non-negative integer) and returns either a normal completion
containingunused or a throw
completion. It allocates and associates an ArrayBuffer with
O. It performs the following steps when called:
does not have a [[ViewedArrayBuffer]] or any other of the internal
slots that are specific to TypedArray instance objects.
23.2.7.1TypedArray.prototype.BYTES_PER_ELEMENT
The value of TypedArray.prototype.BYTES_PER_ELEMENT is the Element
Size value specified in Table 75 for
TypedArray.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
23.2.7.2TypedArray.prototype.constructor
The initial value of the "constructor" property of the prototype for a
given TypedArrayconstructor is the constructor itself.
23.2.8 Properties of TypedArray Instances
TypedArray instances are TypedArrays. Each TypedArray instance
inherits properties from the corresponding TypedArray prototype object. Each
TypedArray instance has the following internal slots: [[ViewedArrayBuffer]], [[TypedArrayName]], [[ContentType]], [[ByteLength]], [[ByteOffset]], and [[ArrayLength]].
24 Keyed Collections
24.1 Map Objects
Maps are collections of key/value pairs where both the keys and values may be arbitrary ECMAScript language values. A distinct
key value may only occur in one key/value pair within the Map's collection. Distinct key values are
discriminated using the semantics of the SameValueZero comparison algorithm.
Maps must be implemented using either hash tables or other mechanisms that, on average, provide
access times that are sublinear on the number of elements in the collection. The data structure used
in this specification is only intended to describe the required observable semantics of Maps. It is
not intended to be a viable implementation model.
is the initial value of the "Map" property of the global
object.
creates and initializes a new Map when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Map behaviour must include a super call to the Map constructor to
create and initialize the subclass instance with the internal state necessary to support the
Map.prototype built-in methods.
24.1.1.1 Map ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
If the parameter iterable is present, it is expected to be an object that
implements a %Symbol.iterator% method that
returns an iterator object that produces
a two element array-like object whose first
element is a value that will be used as a Map key and whose second element is the
value to associate with that key.
The parameter iterable is expected to be an object that implements a
%Symbol.iterator% method that
returns an iterator object that produces
a two element array-like object whose first
element is a value that will be used as a Map key and whose second element is the
value to associate with that key.
callback should be a function that accepts two arguments.
groupBy calls callback once for each element in
items, in ascending order, and constructs a new Map. Each value returned
by callback is used as a key in the Map. For each such key, the result
Map has an entry whose key is that key and whose value is an array containing all
the elements for which callback returned that key.
callback is called with two arguments: the value of the element and the
index of the element.
The return value of groupBy is a Map.
This function performs the following steps when called:
Let groups be ? GroupBy(items,
callback, collection).
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
24.1.2.3 get Map [ %Symbol.species% ]
Map[%Symbol.species%] is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
For each Record { [[Key]], [[Value]] }
p of M.[[MapData]], do
Set p.[[Key]] to
empty.
Set p.[[Value]] to
empty.
Return undefined.
Note
The existing [[MapData]]List is
preserved because there may be existing Map Iterator
objects that are suspended midway through iterating over that
List.
24.1.3.2 Map.prototype.constructor
The initial value of Map.prototype.constructor is %Map%.
24.1.3.3 Map.prototype.delete ( key )
This method performs the following steps when called:
For each Record { [[Key]], [[Value]] }
p of M.[[MapData]], do
If p.[[Key]] is not
empty and SameValue(p.[[Key]], key) is true,
then
Set p.[[Key]] to
empty.
Set p.[[Value]] to
empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate
that an entry has been deleted. Actual implementations may take other actions such
as physically removing the entry from internal data structures.
24.1.3.4 Map.prototype.entries ( )
This method performs the following steps when called:
If IsCallable(callback) is
false, throw a TypeError exception.
Let entries be M.[[MapData]].
Let numEntries be the number of elements in entries.
Let index be 0.
Repeat, while index < numEntries,
Let e be entries[index].
Set index to index + 1.
If e.[[Key]] is not
empty, then
Perform ? Call(callback,
thisArg, « e.[[Value]], e.[[Key]], M »).
NOTE: The number of elements in entries may have
increased during execution of callback.
Set numEntries to the number of elements in
entries.
Return undefined.
Note
callback should be a function that accepts three arguments.
forEach calls callback once for each key/value pair present
in the Map, in key insertion order. callback is called only for keys of
the Map which actually exist; it is not called for keys that have been deleted from
the Map.
If a thisArg parameter is provided, it will be used as the
this value for each invocation of callback. If it is
not provided, undefined is used instead.
callback is called with three arguments: the value of the item, the key of
the item, and the Map being traversed.
forEach does not directly mutate the object on which it is called but
the object may be mutated by the calls to callback. Each entry of a map's
[[MapData]] is only visited once. New keys added after the
call to forEach begins are visited. A key will be revisited if it is
deleted after it has been visited and then re-added before the forEach
call completes. Keys that are deleted after the call to forEach begins
and before being visited are not visited unless the key is added again before the
forEach call completes.
24.1.3.6 Map.prototype.get ( key )
This method performs the following steps when called:
For each Record { [[Key]], [[Value]] }
p of M.[[MapData]], do
If p.[[Key]] is not
empty and SameValue(p.[[Key]], key) is true,
then
Set p.[[Value]] to
value.
Return M.
Let p be the Record { [[Key]]: key, [[Value]]:
value }.
Append p to M.[[MapData]].
Return M.
24.1.3.10 get Map.prototype.size
Map.prototype.size is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
The initial value of the %Symbol.iterator% property is
%Map.prototype.entries%, defined in 24.1.3.4.
24.1.3.13 Map.prototype [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the
String value "Map".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.1.4 Properties of Map Instances
Map instances are ordinary objects that inherit properties from the
Map prototype. Map instances also have a [[MapData]] internal slot.
24.1.5 Map Iterator Objects
A Map
Iterator is an object that represents a specific iteration over some specific Map
instance object. There is not a named constructor for Map Iterator objects. Instead, Map
Iterator objects are created by calling certain methods of Map instance objects.
The initial value of the %Symbol.toStringTag% property is
the String value "Map Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
24.2 Set Objects
Set objects are collections of ECMAScript language values. A distinct
value may only occur once as an element of a Set's collection. Distinct values are discriminated
using the semantics of the SameValueZero comparison algorithm.
Set objects must be implemented using either hash tables or other mechanisms that, on average,
provide access times that are sublinear on the number of elements in the collection. The data
structure used in this specification is only intended to describe the required observable semantics
of Set objects. It is not intended to be a viable implementation model.
24.2.1 Abstract Operations For Set Objects
24.2.1.1 Set Records
A Set Record is a Record value used to
encapsulate the interface of a Set or similar object.
If IsCallable(keys) is
false, throw a TypeError exception.
Return a new Set Record { [[SetObject]]: obj, [[Size]]: intSize, [[Has]]: has, [[Keys]]:
keys }.
24.2.1.3 SetDataHas ( setData, value )
The abstract operation SetDataHas takes arguments setData (a List of either
ECMAScript language values or
empty) and value (an ECMAScript language value) and
returns a Boolean. It performs the following steps when called:
If SetDataIndex(setData,
value) is not-found, return
false.
Return true.
24.2.1.4 SetDataIndex ( setData, value )
The abstract operation SetDataIndex takes arguments setData (a List of either
ECMAScript language values or
empty) and value (an ECMAScript language value) and
returns a non-negative integer or not-found. It
performs the following steps when called:
The abstract operation SetDataSize takes argument setData (a List of either
ECMAScript language values or
empty) and returns a non-negative integer. It performs the following
steps when called:
is the initial value of the "Set" property of the global
object.
creates and initializes a new Set object when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Set behaviour must include a super call to the Set constructor to
create and initialize the subclass instance with the internal state necessary to support the
Set.prototype built-in methods.
24.2.2.1 Set ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
24.2.3.2 get Set [ %Symbol.species% ]
Set[%Symbol.species%] is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Replace the element of S.[[SetData]]
whose value is e with an element whose value is
empty.
Return undefined.
Note
The existing [[SetData]]List is
preserved because there may be existing Set Iterator
objects that are suspended midway through iterating over that
List.
24.2.4.3 Set.prototype.constructor
The initial value of Set.prototype.constructor is %Set%.
24.2.4.4 Set.prototype.delete ( value )
This method performs the following steps when called:
If e is not empty and SameValue(e,
value) is true, then
Replace the element of S.[[SetData]] whose value is e with
an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate
that an entry has been deleted. Actual implementations may take other actions such
as physically removing the entry from internal data structures.
24.2.4.5 Set.prototype.difference ( other )
This method performs the following steps when called:
NOTE: The number of elements in entries may have
increased during execution of callback.
Set numEntries to the number of elements in
entries.
Return undefined.
Note
callback should be a function that accepts three arguments.
forEach calls callback once for each value present in the
Set object, in value insertion order. callback is called only for values
of the Set which actually exist; it is not called for keys that have been deleted
from the set.
If a thisArg parameter is provided, it will be used as the
this value for each invocation of callback. If it is
not provided, undefined is used instead.
callback is called with three arguments: the first two arguments are a
value contained in the Set. The same value is passed for both arguments. The Set
object being traversed is passed as the third argument.
The callback is called with three arguments to be consistent with the call
back functions used by forEach methods for Map and Array. For Sets,
each item value is considered to be both the key and the value.
forEach does not directly mutate the object on which it is called but
the object may be mutated by the calls to callback.
Each value is normally visited only once. However, a value will be revisited if it is
deleted after it has been visited and then re-added before the forEach
call completes. Values that are deleted after the call to forEach
begins and before being visited are not visited unless the value is added again
before the forEach call completes. New values added after the call to
forEach begins are visited.
24.2.4.8 Set.prototype.has ( value )
This method performs the following steps when called:
If SetDataSize(O.[[SetData]]) ≤ otherRec.[[Size]], then
Let thisSize be the number of elements in O.[[SetData]].
Let index be 0.
Repeat, while index < thisSize,
Let e be O.[[SetData]][index].
Set index to index + 1.
If e is not empty, then
Let inOther be ToBoolean(?
Call(otherRec.[[Has]], otherRec.[[SetObject]], «
e »)).
If inOther is true, then
NOTE: It is possible for earlier calls to
otherRec.[[Has]]
to remove and re-add an element of O.[[SetData]], which can cause
the same element to be visited twice during this
iteration.
The initial value of the "keys" property is %Set.prototype.values%,
defined in 24.2.4.17.
Note
For iteration purposes, a Set appears similar to a Map where each entry has the same
value for its key and value.
24.2.4.14 get Set.prototype.size
Set.prototype.size is an accessor property whose set accessor function
is undefined. Its get accessor function performs the following steps when
called:
The initial value of the %Symbol.toStringTag% property is the
String value "Set".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.2.5 Properties of Set Instances
Set instances are ordinary objects that inherit properties from the
Set prototype. Set instances also have a [[SetData]] internal slot.
24.2.6 Set Iterator Objects
A Set
Iterator is an ordinary object, with the structure defined
below, that represents a specific iteration over some specific Set instance object. There is not
a named constructor for Set Iterator objects. Instead, Set
Iterator objects are created by calling certain methods of Set instance objects.
The initial value of the %Symbol.toStringTag% property is
the String value "Set Iterator".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]:
false, [[Configurable]]:
true }.
24.3 WeakMap Objects
WeakMaps are collections of key/value pairs where the keys are objects and/or symbols and values may
be arbitrary ECMAScript language values. A WeakMap
may be queried to see if it contains a key/value pair with a specific key, but no mechanism is
provided for enumerating the values it holds as keys. In certain conditions, values which are not
live are
removed as WeakMap keys, as described in 9.9.3.
An implementation may impose an arbitrarily determined latency between the time a key/value pair of a
WeakMap becomes inaccessible and the time when the key/value pair is removed from the WeakMap. If
this latency was observable to ECMAScript program, it would be a source of indeterminacy that could
impact program execution. For that reason, an ECMAScript implementation must not provide any means
to observe a key of a WeakMap that does not require the observer to present the observed key.
WeakMaps must be implemented using either hash tables or other mechanisms that, on average, provide
access times that are sublinear on the number of key/value pairs in the collection. The data
structure used in this specification is only intended to describe the required observable semantics
of WeakMaps. It is not intended to be a viable implementation model.
Note
WeakMap and WeakSet are intended to provide mechanisms for dynamically associating state with
an object or symbol in a manner that does not “leak” memory resources if, in the absence of
the WeakMap or WeakSet instance, the object or symbol otherwise became inaccessible and
subject to resource reclamation by the implementation's garbage collection mechanisms. This
characteristic can be achieved by using an inverted per-object/symbol mapping of WeakMap or
WeakSet instances to keys. Alternatively, each WeakMap or WeakSet instance may internally
store its key and value data, but this approach requires coordination between the WeakMap or
WeakSet implementation and the garbage collector. The following references describe
mechanism that may be useful to implementations of WeakMap and WeakSet:
Barry Hayes. 1997. Ephemerons: a new finalization mechanism. In Proceedings of the 12th
ACM SIGPLAN conference on Object-oriented programming, systems, languages, and
applications (OOPSLA '97), A. Michael Berman (Ed.). ACM, New York, NY, USA, 176-183,
http://doi.acm.org/10.1145/263698.263733.
is the initial value of the "WeakMap" property of the global
object.
creates and initializes a new WeakMap when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
WeakMap behaviour must include a super call to the WeakMap constructor to
create and initialize the subclass instance with the internal state necessary to support the
WeakMap.prototype built-in methods.
24.3.1.1 WeakMap ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
If the parameter iterable is present, it is expected to be an object that
implements a %Symbol.iterator% method that
returns an iterator object that produces
a two element array-like object whose first
element is a value that will be used as a WeakMap key and whose second element is
the value to associate with that key.
For each Record { [[Key]], [[Value]] }
p of M.[[WeakMapData]], do
If p.[[Key]] is not
empty and SameValue(p.[[Key]], key) is true,
then
Set p.[[Key]] to
empty.
Set p.[[Value]] to
empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate
that an entry has been deleted. Actual implementations may take other actions such
as physically removing the entry from internal data structures.
24.3.3.3 WeakMap.prototype.get ( key )
This method performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the
String value "WeakMap".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.3.4 Properties of WeakMap Instances
WeakMap instances are ordinary objects that inherit properties from the
WeakMap prototype. WeakMap instances also have a [[WeakMapData]]
internal slot.
24.4 WeakSet Objects
WeakSets are collections of objects and/or symbols. A distinct object or symbol may only occur once
as an element of a WeakSet's collection. A WeakSet may be queried to see if it contains a specific
value, but no mechanism is provided for enumerating the values it holds. In certain conditions,
values which are not live are removed as WeakSet elements, as described in
9.9.3.
An implementation may impose an arbitrarily determined latency between the time a value contained in
a WeakSet becomes inaccessible and the time when the value is removed from the WeakSet. If this
latency was observable to ECMAScript program, it would be a source of indeterminacy that could
impact program execution. For that reason, an ECMAScript implementation must not provide any means
to determine if a WeakSet contains a particular value that does not require the observer to present
the observed value.
WeakSets must be implemented using either hash tables or other mechanisms that, on average, provide
access times that are sublinear on the number of elements in the collection. The data structure used
in this specification is only intended to describe the required observable semantics of WeakSets. It
is not intended to be a viable implementation model.
is the initial value of the "WeakSet" property of the global
object.
creates and initializes a new WeakSet when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
WeakSet behaviour must include a super call to the WeakSet constructor to
create and initialize the subclass instance with the internal state necessary to support the
WeakSet.prototype built-in methods.
24.4.1.1 WeakSet ( [ iterable ] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
If e is not empty and SameValue(e,
value) is true, then
Replace the element of S.[[WeakSetData]] whose value is e
with an element whose value is empty.
Return true.
Return false.
Note
The value empty is used as a specification device to indicate
that an entry has been deleted. Actual implementations may take other actions such
as physically removing the entry from internal data structures.
24.4.3.4 WeakSet.prototype.has ( value )
This method performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the
String value "WeakSet".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
24.4.4 Properties of WeakSet Instances
WeakSet instances are ordinary objects that inherit properties from the
WeakSet prototype. WeakSet instances also have a [[WeakSetData]]
internal slot.
The descriptions below in this section, 25.4, and 29 use the read-modify-write
modification function internal data structure.
A read-modify-write
modification function is a mathematical function that is represented as an abstract
closure that takes two Lists of
byte
values as arguments and returns a List of byte
values. These abstract closures satisfy all of the following properties:
They perform all their algorithm steps atomically.
Their individual algorithm steps are not observable.
Note
To aid verifying that a read-modify-write modification function's algorithm steps
constitute a pure, mathematical function, the following editorial conventions are
recommended:
They do not access, directly or transitively via invoked abstract
operations and abstract closures, any language or
specification values except their parameters and captured values.
The abstract operation AllocateArrayBuffer takes arguments constructor (a
constructor) and byteLength (a
non-negative integer) and optional argument
maxByteLength (a non-negative integer or empty) and returns
either a normal completion
containing an ArrayBuffer or a throw completion. It
is used to create an ArrayBuffer. It performs the following steps when called:
Let slots be « [[ArrayBufferData]], [[ArrayBufferByteLength]], [[ArrayBufferDetachKey]] ».
If maxByteLength is present and maxByteLength is not
empty, let allocatingResizableBuffer be
true; otherwise let allocatingResizableBuffer be
false.
If allocatingResizableBuffer is true, then
If byteLength > maxByteLength, throw a
RangeError exception.
If it is not possible to create a Data Blockblock consisting of maxByteLength bytes, throw a
RangeError exception.
NOTE: Resizable ArrayBuffers are designed to be implementable with in-place
growth. Implementations may throw if, for example, virtual memory cannot be
reserved up front.
Set obj.[[ArrayBufferMaxByteLength]] to
maxByteLength.
Return obj.
25.1.3.2 ArrayBufferByteLength ( arrayBuffer,
order )
The abstract operation ArrayBufferByteLength takes arguments arrayBuffer (an
ArrayBuffer or SharedArrayBuffer) and order (seq-cst or
unordered) and returns a non-negative integer. It performs the following
steps when called:
If IsSharedArrayBuffer(arrayBuffer)
is true and arrayBuffer has an [[ArrayBufferByteLengthData]] internal slot, then
Let bufferByteLengthBlock be arrayBuffer.[[ArrayBufferByteLengthData]].
NOTE: Neither creation of the new Data Block nor copying
from the old Data Block are observable.
Implementations may implement this method as a zero-copy move or a
realloc.
The abstract operation IsDetachedBuffer takes argument arrayBuffer (an ArrayBuffer
or a SharedArrayBuffer) and returns a Boolean. It performs the following steps when called:
If arrayBuffer.[[ArrayBufferData]] is
null, return true.
The abstract operation DetachArrayBuffer takes argument arrayBuffer (an
ArrayBuffer) and optional argument key (anything) and returns either a normal completion
containingunused or a throw completion. It
performs the following steps when called:
If arrayBuffer.[[ArrayBufferDetachKey]] is not
key, throw a TypeError exception.
Set arrayBuffer.[[ArrayBufferData]] to
null.
Set arrayBuffer.[[ArrayBufferByteLength]] to 0.
Return unused.
Note
Detaching an ArrayBuffer instance disassociates the Data Block used as its
backing store from the instance and sets the byte length of the buffer to 0.
The abstract operation CloneArrayBuffer takes arguments srcBuffer (an ArrayBuffer
or a SharedArrayBuffer), srcByteOffset (a non-negative integer), and srcLength (a
non-negative integer) and returns either a normal completion
containing an ArrayBuffer or a throw completion. It
creates a new ArrayBuffer whose data is a copy of srcBuffer's data over the range
starting at srcByteOffset and continuing for srcLength bytes. It
performs the following steps when called:
The host-defined abstract operation
HostResizeArrayBuffer takes arguments buffer (an ArrayBuffer) and
newByteLength (a non-negative integer) and returns either a normal completion
containing either handled or
unhandled, or a throw completion. It
gives the host an
opportunity to perform implementation-defined resizing of
buffer. If the host chooses not to handle resizing of
buffer, it may return unhandled for the default behaviour.
The implementation of HostResizeArrayBuffer must conform to the following requirements:
The abstract operation does not detach buffer.
If the abstract operation completes normally with handled,
buffer.[[ArrayBufferByteLength]] is
newByteLength.
The default implementation of HostResizeArrayBuffer is to return NormalCompletion(unhandled).
25.1.3.9 IsFixedLengthArrayBuffer ( arrayBuffer )
The abstract operation IsFixedLengthArrayBuffer takes argument arrayBuffer (an
ArrayBuffer or a SharedArrayBuffer) and returns a Boolean. It performs the following steps
when called:
If arrayBuffer has an [[ArrayBufferMaxByteLength]] internal slot, return
false.
Return true.
25.1.3.10 IsUnsignedElementType ( type )
The abstract operation IsUnsignedElementType takes argument type (a TypedArray element type) and returns a
Boolean. It verifies if the argument type is an unsigned TypedArray element type. It performs
the following steps when called:
If type is one of uint8,
uint8clamped, uint16,
uint32, or biguint64, return
true.
Return false.
25.1.3.11 IsUnclampedIntegerElementType ( type )
The abstract operation IsUnclampedIntegerElementType takes argument type (a
TypedArray element type) and returns a
Boolean. It verifies if the argument type is an IntegerTypedArray element type not including
uint8clamped. It performs the following steps when called:
If type is one of int8,
uint8, int16,
uint16, int32, or
uint32, return true.
If type is either biguint64 or
bigint64, return true.
Return false.
25.1.3.13 IsNoTearConfiguration ( type,
order )
The abstract operation IsNoTearConfiguration takes arguments type (a TypedArray element type) and
order (seq-cst, unordered, or
init) and returns a Boolean. It performs the following steps when
called:
The abstract operation RawBytesToNumeric takes arguments type (a TypedArray element type),
rawBytes (a List of
byte
values), and isLittleEndian (a Boolean) and returns a
Number or a BigInt. It performs the following steps when called:
Let elementSize be the Element Size value specified in Table 75 for
Element Type type.
If isLittleEndian is false, reverse the order of the
elements of rawBytes.
If type is float16, then
Let value be the byte elements of rawBytes
concatenated and interpreted as a little-endian bit string encoding of an
IEEE 754-2019 binary16
value.
If value is a NaN, return NaN.
Return the Number value that corresponds to value.
If type is float32, then
Let value be the byte elements of rawBytes
concatenated and interpreted as a little-endian bit string encoding of an
IEEE 754-2019 binary32
value.
If value is a NaN, return NaN.
Return the Number value that corresponds to value.
If type is float64, then
Let value be the byte elements of rawBytes
concatenated and interpreted as a little-endian bit string encoding of an
IEEE 754-2019 binary64
value.
If value is a NaN, return NaN.
Return the Number value that corresponds to value.
Let intValue be the byte elements of rawBytes
concatenated and interpreted as a bit string encoding of an unsigned
little-endian binary number.
Else,
Let intValue be the byte elements of rawBytes
concatenated and interpreted as a bit string encoding of a binary
little-endian two's complement number of bit length elementSize ×
8.
If IsBigIntElementType(type)
is true, return the BigInt value that corresponds to
intValue.
Otherwise, return the Number value that corresponds to intValue.
25.1.3.15 GetRawBytesFromSharedBlock ( block,
byteIndex, type, isTypedArray, order )
The abstract operation GetRawBytesFromSharedBlock takes arguments block (a
Shared Data
Block), byteIndex (a non-negative integer),
type (a TypedArray element type),
isTypedArray (a Boolean), and order (seq-cst or
unordered) and returns a List of byte
values. It performs the following steps when called:
Let elementSize be the Element Size value specified in Table 75 for
Element Type type.
If isTypedArray is true and IsNoTearConfiguration(type,
order) is true, let noTear be
true; otherwise let noTear be
false.
Let rawValue be a List of length
elementSize whose elements are nondeterministically chosen byte
values.
NOTE: In implementations, rawValue is the result of a non-atomic or
atomic read instruction on the underlying hardware. The nondeterminism is a semantic
prescription of the memory model to describe observable
behaviour of hardware with weak consistency.
Let readEvent be ReadSharedMemory { [[Order]]: order, [[NoTear]]: noTear, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize }.
Append readEvent to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: readEvent, [[ChosenValue]]: rawValue } to
execution.[[ChosenValues]].
The abstract operation GetValueFromBuffer takes arguments arrayBuffer (an
ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer),
type (a TypedArray element type),
isTypedArray (a Boolean), and order (seq-cst or
unordered) and optional argument isLittleEndian (a
Boolean) and returns a Number or a BigInt. It performs the following steps when called:
Let rawValue be a List
whose elements are bytes from block at indices in the interval from byteIndex
(inclusive) to byteIndex + elementSize (exclusive).
Assert:
The number of elements in rawValue is elementSize.
If isLittleEndian is not present, set isLittleEndian to the
value of the [[LittleEndian]] field of the surrounding agent's Agent
Record.
The abstract operation NumericToRawBytes takes arguments type (a TypedArray element type),
value (a Number or a BigInt), and isLittleEndian (a Boolean) and
returns a List of byte
values. It performs the following steps when called:
If type is float16, then
Let rawBytes be a List
whose elements are the 2 bytes that are the result of converting
value to IEEE 754-2019 binary16
format using roundTiesToEven mode. The bytes are arranged in little endian
order. If value is NaN, rawBytes
may be set to any implementation chosen IEEE 754-2019
binary16 format NaN encoding. An implementation must always choose the same
encoding for each implementation distinguishable NaN
value.
Else if type is float32, then
Let rawBytes be a List
whose elements are the 4 bytes that are the result of converting
value to IEEE 754-2019 binary32
format using roundTiesToEven mode. The bytes are arranged in little endian
order. If value is NaN, rawBytes
may be set to any implementation chosen IEEE 754-2019
binary32 format NaN encoding. An implementation must always choose the same
encoding for each implementation distinguishable NaN
value.
Else if type is float64, then
Let rawBytes be a List
whose elements are the 8 bytes that are the IEEE 754-2019 binary64
format encoding of value. The bytes are arranged in little endian
order. If value is NaN, rawBytes
may be set to any implementation chosen IEEE 754-2019
binary64 format NaN encoding. An implementation must always choose the same
encoding for each implementation distinguishable NaN
value.
Else,
Let n be the Element Size value specified in Table 75
for Element Type type.
Let conversionOperation be the abstract operation named in the
Conversion Operation column in Table 75
for Element Type type.
The abstract operation SetValueInBuffer takes arguments arrayBuffer (an
ArrayBuffer or SharedArrayBuffer), byteIndex (a non-negative integer),
type (a TypedArray element type),
value (a Number or a BigInt), isTypedArray (a Boolean), and
order (seq-cst, unordered, or
init) and optional argument isLittleEndian (a Boolean) and
returns unused. It performs the following steps when called:
Store the individual bytes of rawBytes into block,
starting at block[byteIndex].
Return unused.
25.1.3.19 GetModifySetValueInBuffer ( arrayBuffer,
byteIndex, type, value, op )
The abstract operation GetModifySetValueInBuffer takes arguments arrayBuffer (an
ArrayBuffer or a SharedArrayBuffer), byteIndex (a non-negative integer),
type (a TypedArray element type),
value (a Number or a BigInt), and op (a read-modify-write modification
function) and returns a Number or a BigInt. It performs the following
steps when called:
Let rawBytesRead be a List
of length elementSize whose elements are nondeterministically
chosen byte values.
NOTE: In implementations, rawBytesRead is the result of a
load-link, of a load-exclusive, or of an operand of a read-modify-write
instruction on the underlying hardware. The nondeterminism is a semantic
prescription of the memory model to describe
observable behaviour of hardware with weak consistency.
Let rmwEvent be ReadModifyWriteSharedMemory
{ [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndex, [[ElementSize]]: elementSize, [[Payload]]: rawBytes, [[ModifyOp]]: op }.
Append rmwEvent to eventsRecord.[[EventList]].
Append Chosen Value Record
{ [[Event]]: rmwEvent, [[ChosenValue]]: rawBytesRead } to
execution.[[ChosenValues]].
Else,
Let rawBytesRead be a List
of length elementSize whose elements are the sequence of
elementSize bytes starting with
block[byteIndex].
Let rawBytesModified be op(rawBytesRead,
rawBytes).
Store the individual bytes of rawBytesModified into
block, starting at block[byteIndex].
is the initial value of the "ArrayBuffer" property of the global
object.
creates and initializes a new ArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
ArrayBuffer behaviour must include a super call to the ArrayBuffer constructor to
create and initialize subclass instances with the internal state necessary to support the
ArrayBuffer.prototype built-in methods.
25.1.4.1 ArrayBuffer ( length [ , options
] )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
25.1.5.3 get ArrayBuffer [ %Symbol.species% ]
ArrayBuffer[%Symbol.species%] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.1.6.1 get ArrayBuffer.prototype.byteLength
ArrayBuffer.prototype.byteLength is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
The initial value of ArrayBuffer.prototype.constructor is %ArrayBuffer%.
25.1.6.3 get ArrayBuffer.prototype.detached
ArrayBuffer.prototype.detached is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
ArrayBuffer.prototype.maxByteLength is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
ArrayBuffer.prototype.resizable is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
NOTE: Neither creation of the new Data Block nor copying
from the old Data Block are observable.
Implementations may implement this method as in-place growth or shrinkage.
Set O.[[ArrayBufferData]] to
newBlock.
Set O.[[ArrayBufferByteLength]] to
newByteLength.
Return undefined.
25.1.6.7 ArrayBuffer.prototype.slice ( start,
end )
This method performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the
String value "ArrayBuffer".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
25.1.7 Properties of ArrayBuffer Instances
ArrayBuffer instances inherit properties from the ArrayBuffer prototype
object. ArrayBuffer instances each have an [[ArrayBufferData]] internal slot, an [[ArrayBufferByteLength]] internal slot, and an [[ArrayBufferDetachKey]] internal slot. ArrayBuffer instances which are
resizable each have an [[ArrayBufferMaxByteLength]] internal slot.
ArrayBuffer instances whose [[ArrayBufferData]] is
null are considered to be detached and all operators to access or modify data
contained in the ArrayBuffer instance will fail.
ArrayBuffer instances whose [[ArrayBufferDetachKey]] is set to a value
other than undefined need to have all DetachArrayBuffer calls
passing that same "detach key" as an argument, otherwise a TypeError will result. This internal
slot is only ever set by certain embedding environments, not by algorithms in this
specification.
25.1.8 Resizable ArrayBuffer Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with resizable
ArrayBuffer.
We recommend that programs be tested in their deployment environments where possible. The
amount of available physical memory differs greatly between hardware devices. Similarly,
virtual memory subsystems also differ greatly between hardware devices as well as
operating systems. An application that runs without out-of-memory errors on a 64-bit
desktop web browser could run out of memory on a 32-bit mobile web browser.
When choosing a value for the "maxByteLength" option for resizable
ArrayBuffer, we recommend that the smallest possible size for the
application be chosen. We recommend that "maxByteLength" does not
exceed 1,073,741,824 (230 bytes or 1GiB).
Please note that successfully constructing a resizable
ArrayBuffer for a particular maximum size does not guarantee that
future resizes will succeed.
Note 2
The following are guidelines for ECMAScript implementers implementing resizable
ArrayBuffer.
Resizable
ArrayBuffer can be implemented as copying upon resize, as
in-place growth via reserving virtual memory up front, or as a combination of both for
different values of the constructor's
"maxByteLength" option.
If a host is
multi-tenanted (i.e. it runs many ECMAScript applications simultaneously), such as a web
browser, and its implementations choose to implement in-place growth by reserving
virtual memory, we recommend that both 32-bit and 64-bit implementations throw for
values of "maxByteLength" ≥ 1GiB to 1.5GiB. This is to reduce the
likelihood a single application can exhaust the virtual memory address space and to
reduce interoperability risk.
If a host does not
have virtual memory, such as those running on embedded devices without an MMU, or if a
host only
implements resizing by copying, it may accept any Number value
for the "maxByteLength" option. However, we
recommend a RangeError be thrown if a memory block of the requested
size can never be allocated. For example, if the requested size is greater than the
maximum amount of usable memory on the device.
25.2 SharedArrayBuffer Objects
25.2.1 Fixed-length and Growable SharedArrayBuffer Objects
A fixed-length SharedArrayBuffer is a SharedArrayBuffer whose byte
length cannot change after creation.
The abstract operation AllocateSharedArrayBuffer takes arguments constructor (a
constructor) and byteLength (a
non-negative integer) and optional argument
maxByteLength (a non-negative integer or empty) and returns
either a normal completion
containing a SharedArrayBuffer or a throw completion. It
is used to create a SharedArrayBuffer. It performs the following steps when called:
Let slots be « [[ArrayBufferData]] ».
If maxByteLength is present and maxByteLength is not
empty, let allocatingGrowableBuffer be
true; otherwise let allocatingGrowableBuffer be
false.
If allocatingGrowableBuffer is true, then
If byteLength > maxByteLength, throw a
RangeError exception.
Append [[ArrayBufferByteLengthData]] and [[ArrayBufferMaxByteLength]] to slots.
Set obj.[[ArrayBufferByteLengthData]] to
byteLengthBlock.
Set obj.[[ArrayBufferMaxByteLength]] to
maxByteLength.
Else,
Set obj.[[ArrayBufferByteLength]] to
byteLength.
Return obj.
25.2.2.2 IsSharedArrayBuffer ( obj )
The abstract operation IsSharedArrayBuffer takes argument obj (an ArrayBuffer or a
SharedArrayBuffer) and returns a Boolean. It tests whether an object is an ArrayBuffer, a
SharedArrayBuffer, or a subtype of either. It performs the following steps when called:
The host-defined abstract operation
HostGrowSharedArrayBuffer takes arguments buffer (a SharedArrayBuffer) and
newByteLength (a non-negative integer) and returns either a normal completion
containing either handled or
unhandled, or a throw completion. It
gives the host an
opportunity to perform implementation-defined growing of
buffer. If the host chooses not to handle growing of buffer,
it may return unhandled for the default behaviour.
The implementation of HostGrowSharedArrayBuffer must conform to the following requirements:
If the abstract operation does not complete normally with
unhandled, and newByteLength < the current byte
length of the buffer or newByteLength > buffer.[[ArrayBufferMaxByteLength]], throw a
RangeError exception.
Let isLittleEndian be the value of the [[LittleEndian]] field of the surrounding
agent's Agent Record. If the abstract operation
completes normally with handled, a WriteSharedMemory or
ReadModifyWriteSharedMemory
event whose [[Order]] is seq-cst, [[Payload]] is NumericToRawBytes(biguint64,
newByteLength, isLittleEndian), [[Block]]
is buffer.[[ArrayBufferByteLengthData]], [[ByteIndex]] is 0, and [[ElementSize]]
is 8 is added to the surrounding agent's candidate execution such that
racing calls to SharedArrayBuffer.prototype.grow are not "lost", i.e.
silently do nothing.
Note
The second requirement above is intentionally vague about how or when the current
byte length of buffer is read. Because the byte length must be updated
via an atomic read-modify-write operation on the underlying hardware, architectures
that use load-link/store-conditional or load-exclusive/store-exclusive instruction
pairs may wish to keep the paired instructions close in the instruction stream. As
such, SharedArrayBuffer.prototype.grow itself does not perform bounds checking on
newByteLength before calling HostGrowSharedArrayBuffer, nor is there a
requirement on when the current byte length is read.
This is in contrast with HostResizeArrayBuffer,
which is guaranteed that the value of newByteLength is ≥ 0 and ≤
buffer.[[ArrayBufferMaxByteLength]].
The default implementation of HostGrowSharedArrayBuffer is to return NormalCompletion(unhandled).
is the initial value of the "SharedArrayBuffer" property of the global
object, if that property is present (see below).
creates and initializes a new SharedArrayBuffer when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
SharedArrayBuffer behaviour must include a super call to the SharedArrayBuffer
constructor to create and initialize subclass
instances with the internal state necessary to support the
SharedArrayBuffer.prototype built-in methods.
Whenever a host does not
provide concurrent access to SharedArrayBuffers it may omit the
"SharedArrayBuffer" property of the global object.
Note
Unlike an ArrayBuffer, a SharedArrayBuffer cannot become
detached, and its internal [[ArrayBufferData]] slot is never
null.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
25.2.4.2 get SharedArrayBuffer [ %Symbol.species% ]
SharedArrayBuffer[%Symbol.species%] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
25.2.5 Properties of the SharedArrayBuffer Prototype Object
does not have an [[ArrayBufferData]] or [[ArrayBufferByteLength]] internal slot.
25.2.5.1 get SharedArrayBuffer.prototype.byteLength
SharedArrayBuffer.prototype.byteLength is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Let byteLengthBlock be O.[[ArrayBufferByteLengthData]].
Let currentByteLengthRawBytes be GetRawBytesFromSharedBlock(byteLengthBlock,
0, biguint64, true,
seq-cst).
Let newByteLengthRawBytes be NumericToRawBytes(biguint64,
ℤ(newByteLength),
isLittleEndian).
Repeat,
NOTE: This is a compare-and-exchange loop to ensure that parallel, racing
grows of the same buffer are totally ordered, are not lost, and do not
silently do nothing. The loop exits if it was able to attempt to grow
uncontended.
Let currentByteLength be ℝ(RawBytesToNumeric(biguint64,
currentByteLengthRawBytes, isLittleEndian)).
If newByteLength = currentByteLength, return
undefined.
If newByteLength < currentByteLength or
newByteLength > O.[[ArrayBufferMaxByteLength]], throw a
RangeError exception.
Let byteLengthDelta be newByteLength -
currentByteLength.
If it is impossible to create a new Shared Data
Block value consisting of byteLengthDelta
bytes, throw a RangeError exception.
NOTE: No new Shared Data Block is
constructed and used here. The observable behaviour of growable
SharedArrayBuffers is specified by allocating a max-sized Shared Data Block at
construction time, and this step captures the requirement that
implementations that run out of memory must throw a
RangeError.
Let readByteLengthRawBytes be AtomicCompareExchangeInSharedBlock(byteLengthBlock,
0, 8, currentByteLengthRawBytes,
newByteLengthRawBytes).
If ByteListEqual(readByteLengthRawBytes,
currentByteLengthRawBytes) is true, return
undefined.
Set currentByteLengthRawBytes to
readByteLengthRawBytes.
Note
Spurious failures of the compare-exchange to update the length are prohibited. If the
bounds checking for the new length passes and the implementation is not out of
memory, a ReadModifyWriteSharedMemory
event (i.e. a successful compare-exchange) is always added into the candidate execution.
Parallel calls to SharedArrayBuffer.prototype.grow are totally ordered. For example,
consider two racing calls: sab.grow(10) and sab.grow(20).
One of the two calls is guaranteed to win the race. The call to
sab.grow(10) will never shrink sab even if
sab.grow(20) happened first; in that case it will instead throw a
RangeError.
25.2.5.4 get SharedArrayBuffer.prototype.growable
SharedArrayBuffer.prototype.growable is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
25.2.5.5 get SharedArrayBuffer.prototype.maxByteLength
SharedArrayBuffer.prototype.maxByteLength is an accessor
property whose set accessor function is undefined.
Its get accessor function performs the following steps when called:
The initial value of the %Symbol.toStringTag% property is the
String value "SharedArrayBuffer".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
25.2.6 Properties of SharedArrayBuffer Instances
SharedArrayBuffer instances inherit properties from the SharedArrayBuffer
prototype object. SharedArrayBuffer instances each have an [[ArrayBufferData]] internal slot. SharedArrayBuffer instances which are
not growable each have an [[ArrayBufferByteLength]] internal slot.
SharedArrayBuffer instances which are growable each have an [[ArrayBufferByteLengthData]] internal slot and an [[ArrayBufferMaxByteLength]] internal slot.
Note
SharedArrayBuffer instances, unlike ArrayBuffer instances, are never detached.
We recommend that programs be tested in their deployment environments where possible. The
amount of available physical memory differ greatly between hardware devices. Similarly,
virtual memory subsystems also differ greatly between hardware devices as well as
operating systems. An application that runs without out-of-memory errors on a 64-bit
desktop web browser could run out of memory on a 32-bit mobile web browser.
When choosing a value for the "maxByteLength" option for growable
SharedArrayBuffer, we recommend that the smallest possible size
for the application be chosen. We recommend that "maxByteLength" does
not exceed 1073741824, or 1GiB.
Please note that successfully constructing a growable
SharedArrayBuffer for a particular maximum size does not
guarantee that future grows will succeed.
Not all loads of a growable
SharedArrayBuffer's length are synchronizing
seq-cst loads. Loads of the length that are for bounds-checking
of an integer-indexed property access, e.g.
u8[idx], are not synchronizing. In general, in the absence of explicit
synchronization, one property access being in-bound does not imply a subsequent property
access in the same agent is also in-bound. In contrast, explicit loads
of the length via the length and byteLength getters on
SharedArrayBuffer, %TypedArray%.prototype,
and DataView.prototype are synchronizing. Loads of the length that are performed by
built-in methods to check if a TypedArray is entirely out-of-bounds are also
synchronizing.
We recommend growable
SharedArrayBuffer be implemented as in-place growth via reserving
virtual memory up front.
Because grow operations can happen in parallel with memory accesses on a growable
SharedArrayBuffer, the constraints of the memory
model require that even unordered accesses do not "tear" (bits of
their values will not be mixed). In practice, this means the underlying data block of a
growable
SharedArrayBuffer cannot be grown by being copied without
stopping the world. We do not recommend stopping the world as an implementation strategy
because it introduces a serialization point and is slow.
Grown memory must appear zeroed from the moment of its creation, including to any racy
accesses in parallel. This can be accomplished via zero-filled-on-demand virtual memory
pages, or careful synchronization if manually zeroing memory.
Integer-indexed property access on
TypedArray views of growable
SharedArrayBuffers is intended to be optimizable similarly to access on TypedArray
views of non-growable SharedArrayBuffers, because integer-indexed property
loads on are not synchronizing on the underlying buffer's length (see programmer
guidelines above). For example, bounds checks for property accesses may still be hoisted
out of loops.
In practice it is difficult to implement growable
SharedArrayBuffer by copying on hosts that do not have virtual
memory, such as those running on embedded devices without an MMU. Memory usage behaviour
of growable SharedArrayBuffers on such hosts may significantly differ from that of
hosts with virtual
memory. Such hosts
should clearly communicate memory usage expectations to users.
25.3 DataView Objects
25.3.1 Abstract Operations For DataView Objects
25.3.1.1 DataView With Buffer Witness Records
A DataView With Buffer
Witness Record is a Record
value used to encapsulate a DataView along with a cached byte length of the viewed buffer.
It is used to help ensure there is a single shared memory read event of the byte length data
block when the viewed buffer is a growable SharedArrayBuffers.
DataView With Buffer Witness Records have the fields listed in Table 77.
The byte length of the object's [[ViewedArrayBuffer]] when the Record
was created.
25.3.1.2 MakeDataViewWithBufferWitnessRecord ( obj,
order )
The abstract operation MakeDataViewWithBufferWitnessRecord takes arguments obj (a
DataView) and order (seq-cst or
unordered) and returns a DataView With Buffer Witness
Record. It performs the following steps when called:
The abstract operation GetViewByteLength takes argument viewRecord (a DataView With Buffer Witness
Record) and returns a non-negative integer. It performs the following
steps when called:
The abstract operation IsViewOutOfBounds takes argument viewRecord (a DataView With Buffer Witness
Record) and returns a Boolean. It performs the following steps when
called:
Let view be viewRecord.[[Object]].
Let bufferByteLength be viewRecord.[[CachedBufferByteLength]].
Assert:
IsDetachedBuffer(view.[[ViewedArrayBuffer]]) is true if and
only if bufferByteLength is detached.
If bufferByteLength is detached, return
true.
Let byteOffsetStart be view.[[ByteOffset]].
If view.[[ByteLength]] is
auto, then
Let byteOffsetEnd be bufferByteLength.
Else,
Let byteOffsetEnd be byteOffsetStart +
view.[[ByteLength]].
If byteOffsetStart > bufferByteLength or
byteOffsetEnd > bufferByteLength, return
true.
NOTE: 0-length DataViews are not considered out-of-bounds.
Return false.
25.3.1.5 GetViewValue ( view,
requestIndex, isLittleEndian, type )
is the initial value of the "DataView" property of the global
object.
creates and initializes a new DataView when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
DataView behaviour must include a super call to the DataView constructor to
create and initialize subclass instances with the internal state necessary to support the
DataView.prototype built-in methods.
does not have a [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], or
[[ByteOffset]] internal slot.
25.3.4.1 get DataView.prototype.buffer
DataView.prototype.buffer is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Assert:
O has a [[ViewedArrayBuffer]] internal slot.
Let buffer be O.[[ViewedArrayBuffer]].
Return buffer.
25.3.4.2 get DataView.prototype.byteLength
DataView.prototype.byteLength is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
DataView.prototype.byteOffset is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
The initial value of the %Symbol.toStringTag% property is the
String value "DataView".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
25.3.5 Properties of DataView Instances
DataView instances are ordinary objects that inherit properties from the
DataView prototype
object. DataView instances each have [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], and [[ByteOffset]] internal
slots.
Note
The value of the [[DataView]] internal slot is not used within
this specification. The simple presence of that internal slot is used within the
specification to identify objects created using the DataView constructor.
25.4 The Atomics Object
The Atomics object:
is %Atomics%.
is the initial value of the "Atomics" property of the global
object.
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
The Atomics object provides functions that operate indivisibly (atomically) on shared memory array
cells as well as functions that let agents wait for and dispatch primitive events. When used with
discipline, the Atomics functions allow multi-agent programs that communicate through shared memory to
execute in a well-understood order even on parallel CPUs. The rules that govern shared-memory
communication are provided by the memory model, defined below.
Note
For informative guidelines for programming and implementing shared memory in ECMAScript,
please see the notes at the end of the memory model section.
25.4.1 Waiter Record
A Waiter Record is a Record value used to
denote a particular call to Atomics.wait or Atomics.waitAsync.
The agent
cluster has a store of WaiterList Records; the store is indexed by
(block, i), where block is a Shared Data Block and i
a byte offset into the memory of block. WaiterList Records are agent-independent: a lookup in
the store of WaiterList Records by (block, i) will result in the same
WaiterList Record in any agent in the agent cluster.
Each WaiterList Record has a critical
section that controls exclusive access to that WaiterList Record during evaluation.
Only a single agent may
enter a WaiterList Record's critical section at one time. Entering and leaving a WaiterList
Record's critical section is controlled by the abstract operationsEnterCriticalSection and LeaveCriticalSection. Operations on a
WaiterList Record—adding and removing waiting agents, traversing the list of agents, suspending and notifying agents on the list, setting
and retrieving the Synchronize event—may only be
performed by agents that
have entered the WaiterList Record's critical section.
The abstract operation RevalidateAtomicAccess takes arguments typedArray (a
TypedArray) and byteIndexInBuffer (an
integer) and
returns either a normal completion
containingunused or a throw completion.
This operation revalidates the index within the backing buffer for atomic operations after
all argument coercions are performed in Atomics methods, as argument coercions can have
arbitrary side effects, which could cause the buffer to become out of bounds. This operation
does not throw when typedArray's backing buffer is a SharedArrayBuffer. It
performs the following steps when called:
If byteIndexInBuffer ≥ taRecord.[[CachedBufferByteLength]], throw a
RangeError exception.
Return unused.
25.4.3.5 GetWaiterList ( block, i )
The abstract operation GetWaiterList takes arguments block (a Shared Data
Block) and i (a non-negative integer that is evenly divisible by 4)
and returns a WaiterList Record. It performs the
following steps when called:
Assert:
i and i + 3 are valid byte offsets within the memory of
block.
Return the WaiterList Record that is
referenced by the pair (block, i).
25.4.3.6 EnterCriticalSection ( WL )
The abstract operation EnterCriticalSection takes argument WL (a WaiterList Record) and returns
unused. It performs the following steps when called:
Append (WL.[[MostRecentLeaveEvent]],
enterEvent) to eventsRecord.[[AgentSynchronizesWith]].
Return unused.
EnterCriticalSection has contention when an agent attempting to enter the critical section must wait for another
agent to leave it.
When there is no contention, FIFO order of EnterCriticalSection calls is observable. When
there is contention, an implementation may choose an arbitrary order but may not cause an
agent to wait
indefinitely.
25.4.3.7 LeaveCriticalSection ( WL )
The abstract operation LeaveCriticalSection takes argument WL (a WaiterList Record) and returns
unused. It performs the following steps when called:
The abstract operation AddWaiter takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
Assert:
There is no Waiter Record in WL.[[Waiters]] whose [[PromiseCapability]] field is waiterRecord.[[PromiseCapability]] and whose [[AgentSignifier]] field is waiterRecord.[[AgentSignifier]].
Append waiterRecord to WL.[[Waiters]].
Return unused.
25.4.3.9 RemoveWaiter ( WL, waiterRecord )
The abstract operation RemoveWaiter takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
The abstract operation RemoveWaiters takes arguments WL (a WaiterList Record) and c (a
non-negative integer or +∞) and returns a List of Waiter
Records. It performs the following steps when called:
Let L be a List whose
elements are the first n elements of WL.[[Waiters]].
Remove the first n elements of WL.[[Waiters]].
Return L.
25.4.3.11 SuspendThisAgent ( WL,
waiterRecord )
The abstract operation SuspendThisAgent takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
Perform LeaveCriticalSection(WL)
and suspend the surrounding agent until the time is
waiterRecord.[[TimeoutTime]], performing the
combined operation in such a way that a notification that arrives after the
critical section is exited but
before the suspension takes effect is not lost. The surrounding agent can only wake
from suspension due to a timeout or due to another agent calling NotifyWaiter with arguments
WL and thisAgent (i.e. via a call to
Atomics.notify).
The abstract operation NotifyWaiter takes arguments WL (a WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
The abstract operation EnqueueResolveInAgentJob takes arguments agentSignifier (an
agent
signifier), promiseCapability (a PromiseCapability Record), and
resolution ("ok" or "timed-out") and
returns unused. It performs the following steps when called:
Let resolveJob be a new JobAbstract Closure
with no parameters that captures agentSignifier,
promiseCapability, and resolution and performs the following
steps when called:
NOTE: There is no special handling of synchronous immediate timeouts.
Asynchronous immediate timeouts have special handling in order to fail fast
and avoid unnecessary Promise jobs.
Let timeoutTime be ℝ(now) + t +
additionalTimeout.
NOTE: When t is +∞, timeoutTime is also +∞.
Let waiterRecord be a new Waiter Record { [[AgentSignifier]]: thisAgent, [[PromiseCapability]]: promiseCapability, [[TimeoutTime]]: timeoutTime, [[Result]]: "ok" }.
additionalTimeout allows implementations to pad timeouts as necessary,
such as for reducing power consumption or coarsening timer resolution to mitigate
timing attacks. This value may differ from call to call of DoWait.
The abstract operation EnqueueAtomicsWaitAsyncTimeoutJob takes arguments WL (a
WaiterList Record) and
waiterRecord (a Waiter Record) and returns
unused. It performs the following steps when called:
Let timeoutJob be a new JobAbstract Closure
with no parameters that captures WL and waiterRecord and
performs the following steps when called:
The abstract operation AtomicCompareExchangeInSharedBlock takes arguments block (a
Shared Data
Block), byteIndexInBuffer (an integer), elementSize (a
non-negative integer), expectedBytes (a List of byte
values), and replacementBytes (a List of byte
values) and returns a List of byte
values. It performs the following steps when called:
Let rawBytesRead be a List of length
elementSize whose elements are nondeterministically chosen byte
values.
NOTE: In implementations, rawBytesRead is the result of a load-link, of a
load-exclusive, or of an operand of a read-modify-write instruction on the
underlying hardware. The nondeterminism is a semantic prescription of the memory
model to describe observable behaviour of hardware with weak
consistency.
NOTE: The comparison of the expected value and the read value is performed outside
of the read-modify-write modification
function to avoid needlessly strong synchronization when the
expected value is not equal to the read value.
If ByteListEqual(rawBytesRead,
expectedBytes) is true, then
Let second be a new read-modify-write
modification function with parameters
(oldBytes, newBytes) that captures nothing and
performs the following steps atomically when called:
Return newBytes.
Let event be ReadModifyWriteSharedMemory
{ [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize, [[Payload]]: replacementBytes, [[ModifyOp]]: second }.
Else,
Let event be ReadSharedMemory
{ [[Order]]: seq-cst, [[NoTear]]: true, [[Block]]: block, [[ByteIndex]]: byteIndexInBuffer, [[ElementSize]]: elementSize }.
Append event to eventsRecord.[[EventList]].
Append Chosen Value Record { [[Event]]: event, [[ChosenValue]]: rawBytesRead } to
execution.[[ChosenValues]].
Return rawBytesRead.
25.4.3.17 AtomicReadModifyWrite ( typedArray,
index, value, op )
The abstract operation ByteListBitwiseOp takes arguments op (&,
^, or |), xBytes (a List of byte
values), and yBytes (a List of byte
values) and returns a List of byte
values. The operation atomically performs a bitwise operation on all
byte
values of the arguments and returns a List of byte
values. It performs the following steps when called:
Assert:
xBytes and yBytes have the same number of elements.
Let resultByte be the result of applying the bitwise
inclusive OR operation to xByte and yByte.
Set i to i + 1.
Append resultByte to result.
Return result.
25.4.3.19 ByteListEqual ( xBytes, yBytes )
The abstract operation ByteListEqual takes arguments xBytes (a List of byte
values) and yBytes (a List of byte
values) and returns a Boolean. It performs the following steps when
called:
If xBytes and yBytes do not have the same number of elements,
return false.
Let i be 0.
For each element xByte of xBytes, do
Let yByte be yBytes[i].
If xByte ≠ yByte, return false.
Set i to i + 1.
Return true.
25.4.4 Atomics.add ( typedArray, index,
value )
This function performs the following steps when called:
Let add be a new read-modify-write modification
function with parameters (xBytes, yBytes)
that captures typedArray and performs the following steps atomically when
called:
This function performs the following steps when called:
Let and be a new read-modify-write modification
function with parameters (xBytes, yBytes)
that captures nothing and performs the following steps atomically when called:
25.4.7 Atomics.exchange ( typedArray, index,
value )
This function performs the following steps when called:
Let second be a new read-modify-write modification
function with parameters (oldBytes,
newBytes) that captures nothing and performs the following steps atomically
when called:
This function is an optimization primitive. The intuition is that if the atomic step of
an atomic primitive (compareExchange, load,
store, add, sub, and,
or, xor, or exchange) on a datum of size
n bytes will be performed without the surrounding agent
acquiring a lock outside the n bytes comprising the datum, then
Atomics.isLockFree(n) will return true.
High-performance algorithms will use this function to determine whether to use locks or
atomic operations in critical sections. If an atomic
primitive is not lock-free then it is often more efficient for an algorithm to provide
its own locking.
Atomics.isLockFree(4) always returns true as that can be
supported on all known relevant hardware. Being able to assume this will generally
simplify programs.
Regardless of the value returned by this function, all atomic operations are guaranteed
to be atomic. For example, they will never have a visible operation take place in the
middle of the operation (e.g., "tearing").
25.4.9 Atomics.load ( typedArray, index )
This function performs the following steps when called:
This function performs the following steps when called:
Let or be a new read-modify-write modification
function with parameters (xBytes, yBytes)
that captures nothing and performs the following steps atomically when called:
This function performs the following steps when called:
Let subtract be a new read-modify-write modification
function with parameters (xBytes, yBytes)
that captures typedArray and performs the following steps atomically when
called:
This function puts the surrounding agent in a wait queue and suspends
it until notified or until the wait times out, returning a String differentiating those cases.
This function performs the following steps when called:
Let xor be a new read-modify-write modification
function with parameters (xBytes, yBytes)
that captures nothing and performs the following steps atomically when called:
does not have a [[Construct]] internal method; it cannot be used as a
constructor
with the new operator.
does not have a [[Call]] internal method; it cannot be invoked as a
function.
The JSON Data Interchange Format is defined in ECMA-404. The JSON interchange format used in this
specification is exactly that described by ECMA-404. Conforming implementations of
JSON.parse and JSON.stringify must support the exact interchange format
described in the ECMA-404 specification without any deletions or extensions to the format.
25.5.1 JSON.parse ( text [ , reviver ] )
This function parses a JSON text (a JSON-formatted String) and produces an ECMAScript language value. The JSON
format represents literals, arrays, and objects with a syntax similar to the syntax for
ECMAScript literals, Array Initializers, and Object Initializers. After parsing, JSON objects
are realized as ECMAScript objects. JSON arrays are realized as ECMAScript Array instances. JSON
strings, numbers, booleans, and null are realized as ECMAScript Strings, Numbers, Booleans, and
null.
The optional reviver parameter is a function that takes two parameters, key
and value. It can filter and transform the results. It is called with each of the
key/value pairs produced by the parse, and its return value is used
instead of the original value. If it returns what it received, the structure is not modified. If
it returns undefined then the property is deleted from the result.
Assert: result is either a String,
a Number, a Boolean, an Object that is defined by either an ArrayLiteral or an
ObjectLiteral, or
null.
Return result.
It is not permitted for a conforming implementation of JSON.parse to extend the
JSON grammars. If an implementation wishes to support a modified or extended JSON
interchange format it must do so by defining a different parse function.
Note 1
Valid JSON text is a subset of the ECMAScript PrimaryExpression syntax. Step
1 verifies that
jsonString conforms to that subset, and step 8 asserts that evaluation
returns a value of an appropriate type.
However, because 13.2.5.5
behaves differently during ParseJSON, the same source text can produce different
results when evaluated as a PrimaryExpression rather than as
JSON. Furthermore, the Early Error for duplicate "__proto__"
properties in object literals, which likewise does not apply during ParseJSON, means
that not all texts accepted by ParseJSON are valid as a PrimaryExpression, despite
matching the grammar.
Note 2
In the case where there are duplicate name Strings within an object, lexically
preceding values for the same key shall be overwritten.
25.5.2 JSON.stringify ( value [ , replacer [ ,
space ] ] )
This function returns a String in UTF-16 encoded JSON format representing an ECMAScript language value, or
undefined. It can take three parameters. The value parameter is an
ECMAScript language value, which is
usually an object or array, although it can also be a String, Boolean, Number or
null. The optional replacer parameter is either a function that
alters the way objects and arrays are stringified, or an array of Strings and Numbers that acts
as an inclusion list for selecting the object properties that will be stringified. The optional
space parameter is a
String or Number that allows the result to have white space injected into
it to improve human readability.
Let state be the JSON Serialization
Record { [[ReplacerFunction]]:
ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]:
gap, [[PropertyList]]:
PropertyList }.
JSON structures are allowed to be nested to any depth, but they must be acyclic. If
value is or contains a cyclic structure, then this function must throw a
TypeError exception. This is an example of a value that cannot be
stringified:
a = [];
a[0] = a;
my_text = JSON.stringify(a); // This must throw a TypeError.
Note 2
Symbolic primitive values are rendered as follows:
The null value is rendered in JSON text as the String value
"null".
The undefined value is not rendered.
The true value is rendered in JSON text as the String value
"true".
The false value is rendered in JSON text as the String value
"false".
Note 3
String values are wrapped in QUOTATION MARK (") code units. The code units
" and \ are escaped with \ prefixes. Control
characters code units are replaced with escape sequences \uHHHH, or with
the shorter forms, \b (BACKSPACE), \f (FORM FEED),
\n (LINE FEED), \r (CARRIAGE RETURN), \t
(CHARACTER TABULATION).
Note 4
Finite numbers
are stringified as if by calling ToString(number).
NaN and Infinity regardless of sign are
represented as the String value "null".
Note 5
Values that do not have a JSON representation (such as undefined and
functions) do not produce a String. Instead they produce the
undefined value. In arrays these values are represented as the String
value "null". In objects an unrepresentable value causes the property
to be excluded from stringification.
Note 6
An object is rendered as U+007B (LEFT CURLY BRACKET) followed by zero or more properties,
separated with a U+002C (COMMA), closed with a U+007D (RIGHT CURLY BRACKET). A property
is a quoted String representing the property name, a U+003A (COLON), and then
the stringified property value. An array is rendered as an opening U+005B (LEFT SQUARE
BRACKET) followed by zero or more values, separated with a U+002C (COMMA), closed with a
U+005D (RIGHT SQUARE BRACKET).
25.5.2.1 JSON Serialization Record
A JSON Serialization Record is
a Record value used to
enable serialization to the JSON format.
JSON Serialization Records have the fields listed in Table 80.
The abstract operation SerializeJSONProperty takes arguments state (a JSON Serialization Record),
key (a String), and holder (an Object) and returns either a normal completion
containing either a String or undefined, or a
throw completion. It
performs the following steps when called:
The abstract operation QuoteJSONString takes argument value (a String) and returns
a String. It wraps value in 0x0022 (QUOTATION MARK) code units and escapes
certain other code units within it. This operation interprets value as a sequence
of UTF-16 encoded code points, as described in 6.1.4. It performs
the following steps when called:
Let product be the String value consisting solely of the code unit 0x0022
(QUOTATION MARK).
If C is listed in the “Code Point” column of Table 81,
then
Set product to the string-concatenation
of product and the escape sequence for C as
specified in the “Escape Sequence” column of the corresponding row.
Set product to the string-concatenation of
product and the code unit 0x0022 (QUOTATION MARK).
Return product.
Table 81: JSON Single Character Escape Sequences
Code Point
Unicode Character Name
Escape Sequence
U+0008
BACKSPACE
\b
U+0009
CHARACTER TABULATION
\t
U+000A
LINE FEED (LF)
\n
U+000C
FORM FEED (FF)
\f
U+000D
CARRIAGE RETURN (CR)
\r
U+0022
QUOTATION MARK
\"
U+005C
REVERSE SOLIDUS
\\
25.5.2.4 UnicodeEscape ( C )
The abstract operation UnicodeEscape takes argument C (a code unit) and returns a
String. It represents C as a Unicode escape sequence. It performs the following
steps when called:
Let properties be the String value formed by
concatenating all the element Strings of partial with
each adjacent pair of Strings separated with the code unit 0x002C
(COMMA). A comma is not inserted either before the first String or
after the last String.
Let separator be the string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED),
and state.[[Indent]].
Let properties be the String value formed by
concatenating all the element Strings of partial with
each adjacent pair of Strings separated with separator.
The separator String is not inserted either before the
first String or after the last String.
Let final be the string-concatenation
of "{", the code unit 0x000A (LINE FEED),
state.[[Indent]],
properties, the code unit 0x000A (LINE FEED),
stepBack, and "}".
Let properties be the String value formed by
concatenating all the element Strings of partial with
each adjacent pair of Strings separated with the code unit 0x002C
(COMMA). A comma is not inserted either before the first String or
after the last String.
Let separator be the string-concatenation
of the code unit 0x002C (COMMA), the code unit 0x000A (LINE FEED),
and state.[[Indent]].
Let properties be the String value formed by
concatenating all the element Strings of partial with
each adjacent pair of Strings separated with separator.
The separator String is not inserted either before the
first String or after the last String.
Let final be the string-concatenation
of "[", the code unit 0x000A (LINE FEED),
state.[[Indent]],
properties, the code unit 0x000A (LINE FEED),
stepBack, and "]".
Remove the last element of state.[[Stack]].
Set state.[[Indent]] to stepBack.
Return final.
Note
The representation of arrays includes only the elements in the interval
from +0𝔽 (inclusive) to array.length
(exclusive). Properties whose keys are not array indices are excluded
from the stringification. An array is stringified as an opening LEFT SQUARE BRACKET,
elements separated by COMMA, and a closing RIGHT SQUARE BRACKET.
25.5.3 JSON [ %Symbol.toStringTag% ]
The initial value of the %Symbol.toStringTag% property is the
String value "JSON".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
26 Managing Memory
26.1 WeakRef Objects
A WeakRef is an object that is used to refer
to a target object or symbol without preserving it from garbage collection. WeakRefs can be dereferenced to allow access
to the target value, if the target hasn't been reclaimed by garbage collection.
is the initial value of the "WeakRef" property of the global
object.
creates and initializes a new WeakRef when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
WeakRef behaviour must include a super call to the
WeakRefconstructor to create and initialize the subclass
instance with the internal state necessary to support the WeakRef.prototype
built-in methods.
26.1.1.1 WeakRef ( target )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
If CanBeHeldWeakly(target)
is false, throw a TypeError exception.
If the WeakRef returns a
target value that is not undefined, then this
target value should not be garbage collected until the current execution
of ECMAScript code has completed. The AddToKeptObjects
operation makes sure read consistency is maintained.
let target = { foo() {} };
let weakRef = newWeakRef(target);
// ... later ...if (weakRef.deref()) {
weakRef.deref().foo();
}
In the above example, if the first deref does not evaluate to
undefined then the second deref cannot either.
The initial value of the %Symbol.toStringTag% property is the
String value "WeakRef".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
26.1.4 WeakRef Abstract Operations
26.1.4.1 WeakRefDeref ( weakRef )
The abstract operation WeakRefDeref takes argument weakRef (a WeakRef) and returns an ECMAScript language value. It
performs the following steps when called:
A FinalizationRegistry is an
object that manages registration and unregistration of cleanup operations that are performed when
target objects and symbols are garbage collected.
is the initial value of the "FinalizationRegistry" property of the
global
object.
creates and initializes a new FinalizationRegistry when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
FinalizationRegistry behaviour must include a super call to the
FinalizationRegistryconstructor to create and initialize the subclass
instance with the internal state necessary to support the
FinalizationRegistry.prototype built-in methods.
26.2.1.1 FinalizationRegistry ( cleanupCallback )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
If IsCallable(cleanupCallback)
is false, throw a TypeError exception.
Let finalizationRegistry be ? OrdinaryCreateFromConstructor(NewTarget,
"%FinalizationRegistry.prototype%", « [[Realm]], [[CleanupCallback]],
[[Cells]] »).
If unregisterToken is not undefined, throw a
TypeError exception.
Set unregisterToken to empty.
Let cell be the Record { [[WeakRefTarget]]: target, [[HeldValue]]: heldValue, [[UnregisterToken]]: unregisterToken }.
Append cell to finalizationRegistry.[[Cells]].
Return undefined.
Note
Based on the algorithms and definitions in this specification, cell.[[HeldValue]] is live when
finalizationRegistry.[[Cells]] contains
cell; however, this does not necessarily mean that cell.[[UnregisterToken]] or cell.[[Target]] are live. For example,
registering an object with itself as its unregister token would not keep the object
alive forever.
An interface is a set of property keys whose associated values match a
specific specification. Any object that provides all the properties as described by an
interface's specification conforms to that interface. An interface is not represented
by a distinct object. There may be many separately implemented objects that conform to any
interface. An individual object may conform to multiple interfaces.
27.1.1.1 The Iterable Interface
The iterable interface includes the property described in Table 82:
The returned object must conform to the IteratorResult
interface. If a previous call to the
next method of an iterator has
returned an IteratorResult
object whose "done" property
is true, then all subsequent calls to the
next method of that object should also return an IteratorResult
object whose "done" property
is true. However, this requirement is not enforced.
Note 1
Arguments may be passed to the next function but their interpretation
and validity is dependent upon the target iterator. The for-of statement and other
common users of iterators do not pass any arguments, so iterator objects that expect
to be used in such a manner must be prepared to deal with being called with no
arguments.
The returned object must conform to the IteratorResult
interface. Invoking this method notifies the
iterator object
that the caller does not intend to make any more next
method calls to the iterator. The
returned IteratorResult
object will typically have a
"done" property whose value is
true, and a "value" property with
the value passed as the argument of the return method.
However, this requirement is not enforced.
The returned object must conform to the IteratorResult
interface. Invoking this method notifies the
iterator object
that the caller has detected an error condition. The argument may be
used to identify the error condition and typically will be an exception
object. A typical response is to throw the value passed as
the argument. If the method does not throw, the returned
IteratorResult
object will typically have a
"done" property whose value is
true.
Note 2
Typically callers of these methods should check for their existence before invoking
them. Certain ECMAScript language features including
for-of, yield*, and array destructuring call
these methods after performing an existence check. Most ECMAScript library functions
that accept iterable objects as arguments
also conditionally call them.
27.1.1.3 The Async Iterable Interface
The async iterable interface includes the properties described in
Table
85:
An object that implements the async iterator interface must include the properties in Table 86. Such objects may also
implement the properties in Table 87.
The returned promise, when fulfilled, must fulfill with an object
that conforms to the IteratorResult
interface. If a previous call to the
next method of an async
iterator has returned a promise for an
IteratorResult
object whose "done"
property is true, then all subsequent calls to
the next method of that object should also return a
promise for an IteratorResult
object whose "done"
property is true. However, this requirement is
not enforced.
Additionally, the IteratorResult
object that serves as a fulfillment value
should have a "value" property whose value is not
a promise (or "thenable"). However, this requirement is also not
enforced.
Note 1
Arguments may be passed to the next function but their interpretation
and validity is dependent upon the target async iterator. The
for-await-of statement and other common users
of async iterators do not pass any arguments, so async iterator objects that expect
to be used in such a manner must be prepared to deal with being called with no
arguments.
The returned promise, when fulfilled, must fulfill with an object
that conforms to the IteratorResult
interface. Invoking this method notifies the
async iterator
object that the caller does not intend to
make any more next method calls to the async
iterator. The returned promise will fulfill
with an IteratorResult
object which will typically have a
"done" property whose value is
true, and a "value" property
with the value passed as the argument of the return
method. However, this requirement is not enforced.
Additionally, the IteratorResult
object that serves as a fulfillment value
should have a "value" property whose value is not
a promise (or "thenable"). If the argument value is used in the
typical manner, then if it is a rejected promise, a promise rejected
with the same reason should be returned; if it is a fulfilled
promise, then its fulfillment value should be used as the
"value" property of the returned promise's
IteratorResult
object fulfillment value. However, these
requirements are also not enforced.
The returned promise, when fulfilled, must fulfill with an object
that conforms to the IteratorResult
interface. Invoking this method notifies the
async iterator
object that the caller has detected an error
condition. The argument may be used to identify the error condition
and typically will be an exception object. A typical response is to
return a rejected promise which rejects with the value passed as the
argument.
If the returned promise is fulfilled, the IteratorResult
object fulfillment value will typically have
a "done" property whose value is
true. Additionally, it should have a
"value" property whose value is not a promise (or
"thenable"), but this requirement is not enforced.
Note 2
Typically callers of these methods should check for their existence before invoking
them. Certain ECMAScript language features including
for-await-of and yield* call
these methods after performing an existence check.
27.1.1.5 The IteratorResult Interface
The IteratorResult
interface includes the properties listed in Table 88:
Table 88: IteratorResult Interface Properties
Property
Value
Requirements
"done"
a Boolean
This is the result status of an iteratornext method call. If the end of the iterator was
reached "done" is true. If the end
was not reached "done" is false
and a value is available. If a "done" property
(either own or inherited) does not exist, it is considered to have the
value false.
If done is false, this is the current iteration
element value. If done is true, this is the return
value of the iterator, if it
supplied one. If the iterator does not
have a return value, "value" is
undefined. In that case, the
"value" property may be absent from the conforming
object if it does not inherit an explicit "value"
property.
27.1.2 Iterator Helper Objects
An Iterator Helper object is an
ordinary
object that represents a lazy transformation of some specific source
iterator object. There is not a named
constructor
for Iterator Helper objects. Instead, Iterator Helper objects are created by calling certain
methods of Iterator instance objects.
NOTE: Once a generator enters the completed state it never leaves it and
its associated execution
context is never resumed. Any execution state
associated with O can be discarded at this point.
All objects defined in this specification that implement the iterator interface also inherit
from %Iterator.prototype%. ECMAScript code may also define objects that inherit from
%Iterator.prototype%. %Iterator.prototype% provides a place where additional methods
that are applicable to all iterator objects may be added.
The following expression is one way that ECMAScript code can access the
%Iterator.prototype% object:
Iterator.prototype.constructor is an accessor property with
attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined
as follows:
27.1.4.1.1 get Iterator.prototype.constructor
The value of the [[Get]] attribute is a built-in function that
requires no arguments. It performs the following steps when called:
Iterator.prototype[%Symbol.toStringTag%] is an accessor property with
attributes { [[Enumerable]]: false, [[Configurable]]: true }. The [[Get]] and [[Set]] attributes are defined
as follows:
27.1.4.14.1 get Iterator.prototype [ %Symbol.toStringTag% ]
The value of the [[Get]] attribute is a built-in function that
requires no arguments. It performs the following steps when called:
Return "Iterator".
27.1.4.14.2 set Iterator.prototype [ %Symbol.toStringTag% ]
The value of the [[Set]] attribute is a built-in function that
takes an argument v. It performs the following steps when called:
All objects defined in this specification that implement the async iterator interface also
inherit from %AsyncIteratorPrototype%. ECMAScript code may also define objects that
inherit from %AsyncIteratorPrototype%. The %AsyncIteratorPrototype% object provides a
place where additional methods that are applicable to all async iterator objects may be
added.
This function performs the following steps when called:
Return the this value.
The value of the "name" property of this function is
"[Symbol.asyncIterator]".
27.1.6 Async-from-Sync Iterator Objects
An Async-from-Sync Iterator
object is an async iterator that adapts a specific
synchronous iterator. Async-from-Sync Iterator objects
are never directly accessible to ECMAScript code. There is not a named constructor for
Async-from-Sync Iterator objects. Instead, Async-from-Sync Iterator objects are created by the
CreateAsyncFromSyncIterator
abstract operation as needed.
The abstract operation CreateAsyncFromSyncIterator takes argument
syncIteratorRecord (an Iterator Record) and returns an
Iterator Record. It is used to create an
async Iterator Record from a synchronous
Iterator Record. It performs the
following steps when called:
The abstract operation AsyncFromSyncIteratorContinuation takes arguments result
(an Object), promiseCapability (a PromiseCapability
Record for an intrinsic %Promise%),
syncIteratorRecord (an Iterator Record), and
closeOnRejection (a Boolean) and returns a Promise. It performs the following
steps when called:
NOTE: Because promiseCapability is derived from the intrinsic %Promise%, the calls to
promiseCapability.[[Reject]] entailed by the use
IfAbruptRejectPromise below
are guaranteed not to throw.
NOTE: onFulfilled is used when processing the "value"
property of an IteratorResult object in
order to wait for its value if it is a promise and re-package the result in a new
"unwrapped" IteratorResult object.
If done is true, or if closeOnRejection is
false, then
Let onRejected be undefined.
Else,
Let closeIterator be a new Abstract
Closure with parameters (error) that
captures syncIteratorRecord and performs the following steps when
called:
A Promise is an object that is used as a placeholder for the eventual results of a deferred (and
possibly asynchronous) computation.
Any Promise is in one of three mutually exclusive states: fulfilled, rejected, and
pending:
A promise p is fulfilled if p.then(f, r) will immediately enqueue a
Job to call the function
f.
A promise p is rejected if p.then(f, r) will immediately enqueue a
Job to call the function
r.
A promise is pending if it is neither fulfilled nor rejected.
A promise is said to be settled if it is not pending, i.e. if it is either fulfilled or
rejected.
A promise is resolved if it is settled or if it has been “locked in” to match the state of
another promise. Attempting to resolve or reject a resolved promise has no effect. A promise is
unresolved if it is not resolved. An unresolved promise is always in the pending state. A
resolved promise may be pending, fulfilled or rejected.
27.2.1 Promise Abstract Operations
27.2.1.1 PromiseCapability Records
A PromiseCapability Record is a
Record value used to
encapsulate a Promise or promise-like object along with the functions that are capable of
resolving or rejecting that promise. PromiseCapability Records are produced by the NewPromiseCapability abstract
operation.
PromiseCapability Records have the fields listed in Table 90.
A PromiseReaction Record is a
Record value used to
store information about how a promise should react when it becomes resolved or rejected with
a given value. PromiseReaction Records are created by the PerformPromiseThen abstract operation,
and are used by the Abstract Closure returned by NewPromiseReactionJob.
PromiseReaction Records have the fields listed in Table 91.
The function that should be applied to the incoming value, and whose
return value will govern what happens to the derived promise. If [[Handler]] is empty, a
function that depends on the value of [[Type]]
will be used instead.
27.2.1.3 CreateResolvingFunctions ( promise )
The abstract operation CreateResolvingFunctions takes argument promise (a Promise)
and returns a Record with fields
[[Resolve]] (a function object) and [[Reject]] (a function object). It performs the following
steps when called:
Let alreadyResolved be the Record { [[Value]]: false }.
The "length" property of a promise resolve function is
1𝔽.
27.2.1.4 FulfillPromise ( promise, value )
The abstract operation FulfillPromise takes arguments promise (a Promise) and
value (an ECMAScript language value) and
returns unused. It performs the following steps when called:
Assert:
The value of promise.[[PromiseState]] is
pending.
Let reactions be promise.[[PromiseFulfillReactions]].
Set promise.[[PromiseResult]] to
value.
Set promise.[[PromiseFulfillReactions]] to
undefined.
Set promise.[[PromiseRejectReactions]] to
undefined.
If IsConstructor(C) is
false, throw a TypeError exception.
NOTE: C is assumed to be a constructor function that
supports the parameter conventions of the Promise constructor (see 27.2.3.1).
Let resolvingFunctions be the Record { [[Resolve]]: undefined, [[Reject]]: undefined }.
Let executorClosure be a new Abstract Closure
with parameters (resolve, reject) that captures
resolvingFunctions and performs the following steps when called:
If resolvingFunctions.[[Resolve]] is not
undefined, throw a TypeError
exception.
If resolvingFunctions.[[Reject]] is not
undefined, throw a TypeError
exception.
If IsCallable(resolvingFunctions.[[Resolve]]) is false, throw a
TypeError exception.
If IsCallable(resolvingFunctions.[[Reject]]) is false, throw a
TypeError exception.
Return the PromiseCapability
Record { [[Promise]]:
promise, [[Resolve]]:
resolvingFunctions.[[Resolve]], [[Reject]]: resolvingFunctions.[[Reject]] }.
Note
This abstract operation supports Promise subclassing, as it is generic on any
constructor that calls a passed executor
function argument in the same way as the Promise constructor. It is used to
generalize static methods of the Promise constructor to any
subclass.
27.2.1.6 IsPromise ( x )
The abstract operation IsPromise takes argument x (an ECMAScript language value) and
returns a Boolean. It checks for the promise brand on an object. It performs the following
steps when called:
If x does not have a [[PromiseState]] internal
slot, return false.
Return true.
27.2.1.7 RejectPromise ( promise, reason )
The abstract operation RejectPromise takes arguments promise (a Promise) and
reason (an ECMAScript language value) and
returns unused. It performs the following steps when called:
Assert:
The value of promise.[[PromiseState]] is
pending.
Let reactions be promise.[[PromiseRejectReactions]].
Set promise.[[PromiseResult]] to
reason.
Set promise.[[PromiseFulfillReactions]] to
undefined.
Set promise.[[PromiseRejectReactions]] to
undefined.
The abstract operation TriggerPromiseReactions takes arguments reactions (a
List of PromiseReaction Records) and
argument (an ECMAScript language value) and
returns unused. It enqueues a new Job for each record in
reactions. Each such Job processes the [[Type]] and
[[Handler]] of the PromiseReaction
Record, and if the [[Handler]] is not
empty, calls it passing the given argument. If the [[Handler]] is empty, the behaviour is
determined by the [[Type]]. It performs the following steps when
called:
The host-defined abstract operation
HostPromiseRejectionTracker takes arguments promise (a Promise) and
operation ("reject" or "handle") and
returns unused. It allows host environments to track
promise rejections.
The default implementation of HostPromiseRejectionTracker is to return
unused.
Note 1
HostPromiseRejectionTracker is called in two scenarios:
When a promise is rejected without any handlers, it is called with its
operation argument set to "reject".
When a handler is added to a rejected promise for the first time, it is called
with its operation argument set to "handle".
A typical implementation of HostPromiseRejectionTracker might try to notify
developers of unhandled rejections, while also being careful to notify them if such
previous notifications are later invalidated by new handlers being attached.
Note 2
If operation is "handle", an implementation should not
hold a reference to promise in a way that would interfere with garbage
collection. An implementation may hold a reference to promise if
operation is "reject", since it is expected that
rejections will be rare and not on hot code paths.
The abstract operation NewPromiseReactionJob takes arguments reaction (a PromiseReaction Record) and
argument (an ECMAScript language value) and
returns a Record with fields
[[Job]] (a JobAbstract Closure) and
[[Realm]] (a Realm Record or null). It
returns a new JobAbstract Closure that applies the
appropriate handler to the incoming value, and uses the handler's return value to resolve or
reject the derived promise associated with that handler. It performs the following steps
when called:
Let job be a new JobAbstract Closure
with no parameters that captures reaction and argument and
performs the following steps when called:
NOTE: handlerRealm is never null unless the
handler is undefined. When the handler is a revoked Proxy
and no ECMAScript code runs, handlerRealm is used to create error
objects.
Return the Record { [[Job]]: job, [[Realm]]:
handlerRealm }.
27.2.2.2 NewPromiseResolveThenableJob (
promiseToResolve, thenable, then )
The abstract operation NewPromiseResolveThenableJob takes arguments
promiseToResolve (a Promise), thenable (an Object), and
then (a JobCallback Record) and returns a
Record with fields
[[Job]] (a JobAbstract Closure) and
[[Realm]] (a Realm Record). It performs the following steps
when called:
Let job be a new JobAbstract Closure
with no parameters that captures promiseToResolve, thenable,
and then and performs the following steps when called:
NOTE: thenRealm is never null. When
then.[[Callback]] is a revoked Proxy and no code
runs, thenRealm is used to create error objects.
Return the Record { [[Job]]: job, [[Realm]]:
thenRealm }.
Note
This Job uses the
supplied thenable and its then method to resolve the given promise.
This process must take place as a Job to ensure that the evaluation of the
then method occurs after evaluation of any surrounding code has
completed.
is the initial value of the "Promise" property of the global
object.
creates and initializes a new Promise when called as a constructor.
is not intended to be called as a function and will throw an exception when called in that
manner.
may be used as the value in an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
Promise behaviour must include a super call to the Promise constructor to
create and initialize the subclass instance with the internal state necessary to support the
Promise and Promise.prototype built-in methods.
27.2.3.1 Promise ( executor )
This function performs the following steps when called:
If NewTarget is undefined, throw a TypeError
exception.
If IsCallable(executor) is
false, throw a TypeError exception.
Let promise be ? OrdinaryCreateFromConstructor(NewTarget,
"%Promise.prototype%", « [[PromiseState]], [[PromiseResult]], [[PromiseFulfillReactions]], [[PromiseRejectReactions]], [[PromiseIsHandled]] »).
Set promise.[[PromiseState]] to
pending.
Set promise.[[PromiseResult]] to
empty.
Set promise.[[PromiseFulfillReactions]] to a new
empty List.
Set promise.[[PromiseRejectReactions]] to a new
empty List.
The executor argument must be a function object. It is
called for initiating and reporting completion of the possibly deferred action
represented by this Promise. The executor is called with two arguments:
resolve and reject. These are functions that may be used by
the executor function to report eventual completion or failure of the
deferred computation. Returning from the executor function does not mean that the
deferred action has been completed but only that the request to eventually perform
the deferred action has been accepted.
The resolve function that is passed to an executor function
accepts a single argument. The executor code may eventually call the
resolve function to indicate that it wishes to resolve the associated
Promise. The argument passed to the resolve function represents the
eventual value of the deferred action and can be either the actual fulfillment value
or another promise which will provide the value if it is fulfilled.
The reject function that is passed to an executor function
accepts a single argument. The executor code may eventually call the
reject function to indicate that the associated Promise is rejected and
will never be fulfilled. The argument passed to the reject function is
used as the rejection value of the promise. Typically it will be an Error object.
The resolve and reject functions passed to an executor function by the
Promise constructor have the capability to
actually resolve and reject the associated promise. Subclasses may have different
constructor behaviour that passes in
customized values for resolve and reject.
This function returns a new promise which is fulfilled with an array of fulfillment values
for the passed promises, or rejects with the reason of the first passed promise that
rejects. It resolves all elements of the passed iterable to promises as
it runs this algorithm.
A Promise.all resolve element function is an anonymous built-in function
that is used to resolve a specific Promise.all element. Each
Promise.all resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]],
and [[AlreadyCalled]] internal slots.
When a Promise.all resolve element function is called with argument
x, the following steps are taken:
The "length" property of a Promise.all resolve element
function is 1𝔽.
27.2.4.2 Promise.allSettled ( iterable )
This function returns a promise that is fulfilled with an array of promise state snapshots,
but only after all the original promises have settled, i.e. become either fulfilled or
rejected. It resolves all elements of the passed iterable to promises as
it runs this algorithm.
27.2.4.2.2Promise.allSettled Resolve Element
Functions
A Promise.allSettled resolve element function is an anonymous built-in
function that is used to resolve a specific Promise.allSettled element.
Each Promise.allSettled resolve element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]],
and [[AlreadyCalled]] internal slots.
When a Promise.allSettled resolve element function is called with argument
x, the following steps are taken:
The "length" property of a Promise.allSettled resolve
element function is 1𝔽.
27.2.4.2.3Promise.allSettled Reject Element
Functions
A Promise.allSettled reject element function is an anonymous built-in
function that is used to reject a specific Promise.allSettled element. Each
Promise.allSettled reject element function has [[Index]], [[Values]], [[Capability]], [[RemainingElements]],
and [[AlreadyCalled]] internal slots.
When a Promise.allSettled reject element function is called with argument
x, the following steps are taken:
The "length" property of a Promise.allSettled reject
element function is 1𝔽.
27.2.4.3 Promise.any ( iterable )
This function returns a promise that is fulfilled by the first given promise to be fulfilled,
or rejected with an AggregateError holding the rejection reasons if all of the
given promises are rejected. It resolves all elements of the passed iterable to promises as it runs this
algorithm.
A Promise.any reject element function is an anonymous built-in function that
is used to reject a specific Promise.any element. Each
Promise.any reject element function has [[Index]],
[[Errors]], [[Capability]], [[RemainingElements]], and [[AlreadyCalled]] internal slots.
When a Promise.any reject element function is called with argument
x, the following steps are taken:
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
27.2.4.5 Promise.race ( iterable )
This function returns a new promise which is settled in the same way as the first passed
promise to settle. It resolves all elements of the passed iterable to promises as
it runs this algorithm.
If the iterable argument yields no values or if none of the promises
yielded by iterable ever settle, then the pending promise returned by
this method will never be settled.
Note 2
This function expects its this value to be a constructor function that supports the
parameter conventions of the Promise constructor. It also
expects that its this value provides a resolve
method.
Perform ? Call(promiseCapability.[[Reject]], undefined, «
r »).
Return promiseCapability.[[Promise]].
Note
This function expects its this value to be a constructor function that supports the
parameter conventions of the Promise constructor.
27.2.4.7 Promise.resolve ( x )
This function returns either a new promise resolved with the passed argument, or the argument
itself if the argument is a promise produced by this constructor.
Promise[%Symbol.species%] is an accessor property whose set
accessor function is undefined. Its get accessor function performs the
following steps when called:
Return the this value.
The value of the "name" property of this function is "get
[Symbol.species]".
Note
Promise prototype methods normally use their this value's
constructor to create a derived object.
However, a subclass constructor may over-ride that default
behaviour by redefining its %Symbol.species% property.
The abstract operation PerformPromiseThen takes arguments promise (a Promise),
onFulfilled (an ECMAScript language
value), and onRejected (an ECMAScript language value)
and optional argument resultCapability (a PromiseCapability Record)
and returns an ECMAScript language value.
It performs the “then” operation on promise using onFulfilled and
onRejected as its settlement actions. If resultCapability is
passed, the result is stored by updating resultCapability's promise. If it is
not passed, then PerformPromiseThen is being called by a specification-internal
operation where the result does not matter. It performs the following steps when called:
creates and initializes a new GeneratorFunction when called as a function rather than as a
constructor. Thus the function call
GeneratorFunction (…) is equivalent to the object creation expression
new GeneratorFunction (…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
GeneratorFunction behaviour must include a super call to the GeneratorFunction
constructor to create and initialize subclass
instances with the internal slots necessary for built-in GeneratorFunction behaviour. All
ECMAScript syntactic forms for defining generator function objects create direct
instances of GeneratorFunction. There is no syntactic means to create instances of
GeneratorFunction subclasses.
The initial value of the %Symbol.toStringTag% property is the
String value "GeneratorFunction".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.3.4 GeneratorFunction Instances
Every GeneratorFunction instance is an ECMAScript function object and has the
internal slots listed in Table
30. The value of the [[IsClassConstructor]]
internal slot for all such instances is false.
Each GeneratorFunction instance has the following own properties:
27.3.4.1 length
The specification for the "length" property of Function instances given in
20.2.4.1 also applies to
GeneratorFunction instances.
27.3.4.2 name
The specification for the "name" property of Function instances given in
20.2.4.2 also applies to
GeneratorFunction instances.
27.3.4.3 prototype
Whenever a GeneratorFunction instance is created another ordinary object is also
created and is the initial value of the generator function's "prototype"
property. The value of the prototype property is used to initialize the [[Prototype]] internal slot of a newly created Generator when the
generator function object is invoked using [[Call]].
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
Unlike Function instances, the object that is the value of a GeneratorFunction's
"prototype" property does not have a
"constructor" property whose value is the GeneratorFunction
instance.
creates and initializes a new AsyncGeneratorFunction when called as a function rather than
as a constructor. Thus the function call
AsyncGeneratorFunction (...) is equivalent to the object creation expression
new AsyncGeneratorFunction (...) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
AsyncGeneratorFunction behaviour must include a super call to the
AsyncGeneratorFunction constructor to create and initialize subclass
instances with the internal slots necessary for built-in AsyncGeneratorFunction behaviour.
All ECMAScript syntactic forms for defining async generator function
objects create direct instances of AsyncGeneratorFunction. There is
no syntactic means to create instances of AsyncGeneratorFunction subclasses.
The initial value of the %Symbol.toStringTag% property is the
String value "AsyncGeneratorFunction".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.4.4 AsyncGeneratorFunction Instances
Every AsyncGeneratorFunction instance is an ECMAScript function object and has the
internal slots listed in Table
30. The value of the [[IsClassConstructor]]
internal slot for all such instances is false.
Each AsyncGeneratorFunction instance has the following own properties:
27.4.4.1 length
The value of the "length" property is an integral Number that indicates
the typical number of arguments expected by the AsyncGeneratorFunction. However, the
language permits the function to be invoked with some other number of arguments. The
behaviour of an AsyncGeneratorFunction when invoked on a number of arguments other than the
number specified by its "length" property depends on the function.
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.4.4.2 name
The specification for the "name" property of Function instances given in
20.2.4.2 also applies to
AsyncGeneratorFunction instances.
27.4.4.3 prototype
Whenever an AsyncGeneratorFunction instance is created, another ordinary
object is also created and is the initial value of the async
generator function's "prototype" property. The value of the prototype
property is used to initialize the [[Prototype]] internal slot of a
newly created AsyncGenerator when the generator function object is invoked
using [[Call]].
This property has the attributes { [[Writable]]:
true, [[Enumerable]]: false,
[[Configurable]]: false }.
Note
Unlike function instances, the object that is the value of an
AsyncGeneratorFunction's "prototype" property does not have a
"constructor" property whose value is the AsyncGeneratorFunction
instance.
Generator instances directly inherit properties from the initial value of the
"prototype" property of the generator function that created the instance.
Generator instances indirectly inherit properties from %GeneratorPrototype%.
The abstract operation GeneratorStart takes arguments generator (a Generator) and
generatorBody (a FunctionBodyParse
Node or an Abstract Closure with no parameters) and
returns unused. It performs the following steps when called:
Assert:
The value of generator.[[GeneratorState]] is
suspended-start.
NOTE: Once a generator enters the completed state it
never leaves it and its associated execution
context is never resumed. Any execution state
associated with acGenerator can be discarded at this point.
The abstract operation GeneratorValidate takes arguments generator (an ECMAScript language value) and
generatorBrand (a String or empty) and returns either a
normal completion
containing one of suspended-start,
suspended-yield, or completed, or a throw completion. It
performs the following steps when called:
Resume the suspended evaluation of
genContext using NormalCompletion(value)
as the result of the operation that suspended it. Let result be the value
returned by the resumed computation.
NOTE: Once a generator enters the completed state it
never leaves it and its associated execution
context is never resumed. Any execution state
associated with generator can be discarded at this point.
Resume the suspended evaluation of
genContext using abruptCompletion as the result of
the operation that suspended it. Let result be the Completion
Record returned by the resumed computation.
Resume callerContext passing NormalCompletion(iteratorResult).
If genContext is ever resumed again, let resumptionValue be
the Completion
Record with which it is resumed.
The abstract operation CreateIteratorFromClosure takes arguments closure (an
Abstract Closure with no parameters),
generatorBrand (a String or empty), and
generatorPrototype (an Object) and optional argument extraSlots (a
List of names of
internal slots) and returns a Generator. It performs the following steps when called:
AsyncGenerator instances directly inherit properties from the initial value of the
"prototype" property of the async generator function that created the instance.
AsyncGenerator instances indirectly inherit properties from %AsyncGeneratorPrototype%.
Records
which represent requests to resume the async generator. Except during state
transitions, it is non-empty if and only if [[AsyncGeneratorState]] is either
executing or draining-queue.
[[GeneratorBrand]]
a String or empty
A brand used to distinguish different kinds of async generators. The [[GeneratorBrand]] of async generators declared by
ECMAScript source text is
always empty.
27.6.3 AsyncGenerator Abstract Operations
27.6.3.1 AsyncGeneratorRequest Records
An AsyncGeneratorRequest is a
Record value used to
store information about how an async generator should be resumed and contains capabilities
for fulfilling or rejecting the corresponding promise.
The abstract operation AsyncGeneratorStart takes arguments generator (an
AsyncGenerator) and generatorBody (a FunctionBodyParse
Node or an Abstract Closure with no parameters) and
returns unused. It performs the following steps when called:
Assert:
generator.[[AsyncGeneratorState]] is
suspended-start.
The abstract operation AsyncGeneratorEnqueue takes arguments generator (an
AsyncGenerator), completion (a Completion Record),
and promiseCapability (a PromiseCapability
Record) and returns unused. It performs the
following steps when called:
Let request be AsyncGeneratorRequest
{ [[Completion]]: completion, [[Capability]]: promiseCapability }.
Append request to generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorCompleteStep takes arguments generator (an
AsyncGenerator), completion (a Completion Record),
and done (a Boolean) and optional argument realm (a Realm
Record) and returns unused. It performs the
following steps when called:
Assert:
generator.[[AsyncGeneratorQueue]] is not empty.
Let next be the first element of generator.[[AsyncGeneratorQueue]].
Remove the first element from generator.[[AsyncGeneratorQueue]].
The abstract operation AsyncGeneratorResume takes arguments generator (an
AsyncGenerator) and completion (a Completion Record)
and returns unused. It performs the following steps when called:
Assert:
generator.[[AsyncGeneratorState]] is either
suspended-start or suspended-yield.
Let genContext be generator.[[AsyncGeneratorContext]].
Resume the suspended evaluation of
genContext using completion as the result of the
operation that suspended it. Let result be the Completion
Record returned by the resumed computation.
The abstract operation AsyncGeneratorAwaitReturn takes argument generator (an
AsyncGenerator) and returns unused. It performs the following steps
when called:
Assert:
generator.[[AsyncGeneratorState]] is
draining-queue.
The abstract operation AsyncGeneratorDrainQueue takes argument generator (an
AsyncGenerator) and returns unused. It drains the generator's
AsyncGeneratorQueue until it encounters an AsyncGeneratorRequest which
holds a return completion.
It performs the following steps when called:
Assert:
generator.[[AsyncGeneratorState]] is
draining-queue.
Let queue be generator.[[AsyncGeneratorQueue]].
Repeat, while queue is not empty,
Let next be the first element of queue.
Let completion be Completion(next.[[Completion]]).
The abstract operation CreateAsyncIteratorFromClosure takes arguments closure (an
Abstract Closure with no parameters),
generatorBrand (a String or empty), and
generatorPrototype (an Object) and returns an AsyncGenerator. It performs the
following steps when called:
creates and initializes a new AsyncFunction when called as a function rather than as a
constructor. Thus the function call
AsyncFunction(…) is equivalent to the object creation expression
new AsyncFunction(…) with the same arguments.
may be used as the value of an extends clause of a class definition. Subclass
constructors that intend to inherit the specified
AsyncFunction behaviour must include a super call to the AsyncFunction
constructor to create and initialize a subclass
instance with the internal slots necessary for built-in async function behaviour. All
ECMAScript syntactic forms for defining async function objects create direct
instances of AsyncFunction. There is no syntactic means to create instances of AsyncFunction
subclasses.
The initial value of the %Symbol.toStringTag% property is the
String value "AsyncFunction".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: true }.
27.7.4 AsyncFunction Instances
Every AsyncFunction instance is an ECMAScript function object and has the
internal slots listed in Table
30. The value of the [[IsClassConstructor]]
internal slot for all such instances is false. AsyncFunction instances are
not constructors and do not have a [[Construct]] internal method. AsyncFunction instances do not have a
prototype property as they are not constructable.
Each AsyncFunction instance has the following own properties:
27.7.4.1 length
The specification for the "length" property of Function instances given in
20.2.4.1 also applies to
AsyncFunction instances.
27.7.4.2 name
The specification for the "name" property of Function instances given in
20.2.4.2 also applies to
AsyncFunction instances.
The abstract operation AsyncBlockStart takes arguments promiseCapability (a
PromiseCapability Record),
asyncBody (a Parse Node or an Abstract
Closure with no parameters), and asyncContext (an
execution context) and returns
unused. It performs the following steps when called:
Set the code evaluation state of asyncContext such that when evaluation
is resumed for that execution context,
closure will be called with no arguments.
Assert:
result is a normal
completion with a value of unused. The
possible sources of this value are Await or, if the async function doesn't await
anything, step 2.i above.
A Module Namespace Object is a module namespace exotic object that
provides runtime property-based access to a module's exported bindings. There is no constructor function
for Module Namespace Objects. Instead, such an object is created for each module that is imported by
an ImportDeclaration that
contains a NameSpaceImport.
In addition to the properties specified in 10.4.6 each Module
Namespace Object has the following own property:
28.3.1 %Symbol.toStringTag%
The initial value of the %Symbol.toStringTag% property is the
String value "Module".
This property has the attributes { [[Writable]]:
false, [[Enumerable]]: false,
[[Configurable]]: false }.
29 Memory Model
The memory consistency model, or memory model, specifies the possible orderings
of Shared Data Block events, arising via
accessing TypedArray
instances backed by a SharedArrayBuffer and via methods on the Atomics object. When the program has no
data races (defined below), the ordering of events appears as sequentially consistent, i.e., as an
interleaving of actions from each agent. When the program has data races, shared memory operations
may appear sequentially inconsistent. For example, programs may exhibit causality-violating behaviour
and other astonishments. These astonishments arise from compiler transforms and the design of CPUs
(e.g., out-of-order execution and speculation). The memory model defines both the precise conditions
under which a program exhibits sequentially consistent behaviour as well as the possible values read
from data races. To wit, there is no undefined behaviour.
The memory model is defined as relational constraints on events introduced by abstract operations on
SharedArrayBuffer or by methods on the Atomics object during an evaluation.
Note
This section provides an axiomatic model on events introduced by the abstract operations
on SharedArrayBuffers. It bears stressing that the model is not expressible algorithmically,
unlike the rest of this specification. The nondeterministic introduction of events by abstract operations
is the interface between the operational semantics of ECMAScript evaluation and the axiomatic
semantics of the memory model. The semantics of these events is defined by considering graphs of
all events in an evaluation. These are neither Static Semantics nor Runtime Semantics. There is
no demonstrated algorithmic implementation, but instead a set of constraints that determine if a
particular event graph is allowed or disallowed.
29.1 Memory Model Fundamentals
Shared memory accesses (reads and writes) are divided into two groups, atomic accesses and data
accesses, defined below. Atomic accesses are sequentially consistent, i.e., there is a strict total
ordering of events agreed upon by all agents in an agent cluster. Non-atomic accesses
do not have a strict total ordering agreed upon by all agents, i.e., unordered.
Note 1
No orderings weaker than sequentially consistent and stronger than unordered, such as
release-acquire, are supported.
A Shared Data Block event is either a
ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory Record.
These events are introduced by abstract
operations or by methods on the Atomics object.
Some operations may also introduce Synchronize events. A Synchronize event has no fields, and exists
purely to directly constrain the permitted orderings of other events.
In addition to Shared
Data Block and Synchronize events, there are host-specific events.
Let the range of a ReadSharedMemory, WriteSharedMemory, or ReadModifyWriteSharedMemory event be the
Set of contiguous integers from its [[ByteIndex]] to
[[ByteIndex]] + [[ElementSize]] - 1. Two events'
ranges are equal when the events have the same [[Block]], and the ranges
are element-wise equal. Two events' ranges are overlapping when the events have the same [[Block]], the ranges are not equal and their intersection is non-empty. Two
events' ranges are disjoint when the events do not have the same [[Block]]
or their ranges are neither equal nor overlapping.
Note 2
Examples of host-specific synchronizing events that should be
accounted for are: sending a SharedArrayBuffer from one agent to another (e.g., by
postMessage in a browser), starting and stopping agents, and communicating within the
agent
cluster via channels other than shared memory. For a particular
execution execution, those events are provided by the host via the host-synchronizes-withstrict partial order.
Additionally, hosts
can add host-specific
synchronizing events to execution.[[EventList]] so as to
participate in the is-agent-order-beforeRelation.
An empty candidate execution is a
candidate execution Record whose fields are empty
Lists.
29.5 Abstract Operations for the Memory Model
29.5.1 EventSet ( execution )
The abstract operation EventSet takes argument execution (a candidate execution) and returns a Set
of events. It performs the following steps when called:
The abstract operation SharedDataBlockEventSet takes argument execution (a candidate execution) and returns a Set
of events. It performs the following steps when called:
The abstract operation HostEventSet takes argument execution (a candidate execution) and returns a Set
of events. It performs the following steps when called:
Let bytesModified be W.[[ModifyOp]](bytes, W.[[Payload]]).
Let byte be
bytesModified[payloadIndex].
Append byte to bytesRead.
Set byteLocation to byteLocation + 1.
Return bytesRead.
Note 1
The read-modify-write modification [[ModifyOp]] is given by the
function properties on the Atomics object that introduce ReadModifyWriteSharedMemory
events.
For events E and D, E is-agent-order-before D in
execution if there is some Agent Events Recordaer in execution.[[EventsRecords]] such that
aer.[[EventList]] contains both E and
D and E is before D in List order of
aer.[[EventList]].
For events R and W, R reads-from W in
execution if SharedDataBlockEventSet(execution)
contains both R and W, and reads-bytes-from(R) in
execution contains W.
For events R and W, W synchronizes-with R in
execution if Rreads-fromW in
execution, R.[[Order]] is
seq-cst, W.[[Order]] is
seq-cst, and R and W have equal ranges.
For each element eventsRecord of execution.[[EventsRecords]], the following is true.
For events S and Sw, S synchronizes-with
Sw in execution if eventsRecord.[[AgentSynchronizesWith]] contains (S,
Sw).
For events E and D, E synchronizes-with D in
execution if execution.[[HostSynchronizesWith]] contains (E, D).
Note 1
Owing to convention in memory model literature, in a candidate executionexecution, write events synchronizes-with read events, instead of read events
synchronizes-with write events.
In a candidate executionexecution, not all seq-cst events related by reads-from are related by
synchronizes-with. Only events that also have equal ranges are related by
synchronizes-with.
Assert: The remainder of dividing
R.[[ByteIndex]] by R.[[ElementSize]] is 0.
For each event W such that Rreads-fromW in
execution and W.[[NoTear]] is true, do
If R and W have equal ranges and there
exists an event V such that V and
W have equal ranges, V.[[NoTear]] is true,
W and V are not the same Shared Data Block
event, and Rreads-fromV in execution, then
Return false.
Return true.
Note
An event's [[NoTear]] field is true when that
event was introduced via accessing an integerTypedArray, and
false when introduced via accessing a floating point TypedArray
or DataView.
Intuitively, this requirement says when a memory range is accessed in an aligned fashion
via an integerTypedArray, a single write
event on that range must "win" when in a data race with other write
events with equal ranges. More precisely, this requirement says an aligned read event
cannot read a value composed of bytes from multiple, different write events all with
equal ranges. It is possible, however, for an aligned read event to read from multiple
write events with overlapping ranges.
For events E and D, E is-memory-order-before D
in execution if Ehappens-beforeD
in execution.
For events R and W such that Rreads-fromW in
execution, there is no WriteSharedMemory or
ReadModifyWriteSharedMemory
event V in SharedDataBlockEventSet(execution)
such that V.[[Order]] is
seq-cst, W is-memory-order-before V in
execution, V is-memory-order-before R in
execution, and any of the following conditions are true.
This clause together with the forward progress guarantee on agents ensure
the liveness condition that seq-cst writes become visible
to seq-cst reads with equal range in finite time.
While is-memory-order-before includes all events in EventSet(execution), those that
are not constrained by happens-before or synchronizes-with in
execution are allowed to occur anywhere in the order.
29.7.5 Valid Executions
A candidate executionexecution
is a valid execution (or simply an execution) if all of the following are true.
For an execution execution and events E and D that are contained in
SharedDataBlockEventSet(execution),
E and D are in a race if the following algorithm returns
true.
For an execution execution and events E and D that are contained in
SharedDataBlockEventSet(execution),
E and D are in a data race if the following algorithm
returns true.
If E.[[Order]] is not
seq-cst or D.[[Order]] is
not seq-cst, then
Return true.
If E and D have overlapping ranges, then
Return true.
Return false.
29.10 Data Race Freedom
An execution execution is data race free if there are no two
events in SharedDataBlockEventSet(execution)
that are in a data
race.
A program is data race free if all its executions are data race free.
The memory
model guarantees sequential consistency of all events for data race free
programs.
29.11 Shared Memory Guidelines
Note 1
The following are guidelines for ECMAScript programmers working with shared memory.
We recommend programs be kept data race free, i.e., make it so that
it is impossible for there to be concurrent non-atomic operations on the same memory
location. Data race free programs have
interleaving semantics where each step in the evaluation semantics of each agent are interleaved with
each other. For data race free programs, it is not
necessary to understand the details of the memory model. The details are
unlikely to build intuition that will help one to better write ECMAScript.
More generally, even if a program is not data race free it may
have predictable behaviour, so long as atomic operations are not involved in any data races
and the operations that race all have the same access size. The simplest way to arrange for
atomics not to be involved in races is to ensure that different memory cells are used by
atomic and non-atomic operations and that atomic accesses of different sizes are not used to
access the same cells at the same time. Effectively, the program should treat shared memory
as strongly typed as much as possible. One still cannot depend on the ordering and timing of
non-atomic accesses that race, but if memory is treated as strongly typed the racing
accesses will not "tear" (bits of their values will not be mixed).
Note 2
The following are guidelines for ECMAScript implementers writing compiler transformations for
programs using shared memory.
It is desirable to allow most program transformations that are valid in a single-agent setting in a
multi-agent setting,
to ensure that the performance of each agent in a multi-agent program is as good as it would be
in a single-agent
setting. Frequently these transformations are hard to judge. We outline some rules about
program transformations that are intended to be taken as normative (in that they are implied
by the memory
model or stronger than what the memory model implies) but
which are likely not exhaustive. These rules are intended to apply to program
transformations that precede the introductions of the events that make up the is-agent-order-beforeRelation.
Let possible read values of a read event be the set of all values of
ValueOfReadEvent for that event across
all valid executions.
Any transformation of an agent-order slice that is valid in the absence of shared memory is
valid in the presence of shared memory, with the following exceptions.
Atomics are carved in stone: Program transformations must not cause the
seq-cst events in an agent-order slice to be reordered with
its unordered operations, nor its
seq-cst operations to be reordered with each other, nor may a
program transformation remove a seq-cst operation from the
is-agent-order-beforeRelation.
(In practice, the prohibition on reorderings forces a compiler to assume that every
seq-cst operation is a synchronization and included in the
final is-memory-order-beforeRelation,
which it would usually have to assume anyway in the absence of inter-agent program
analysis. It also forces the compiler to assume that every call where the callee's
effects on the memory-order are unknown may contain seq-cst
operations.)
Reads must be stable: Any given shared memory read must only observe a
single value in an execution.
(For example, if what is semantically a single read in the program is executed
multiple times then the program is subsequently allowed to observe only one of the
values read. A transformation known as rematerialization can violate this rule.)
Writes must be stable: All observable writes to shared memory must follow
from program semantics in an execution.
(For example, a transformation may not introduce certain observable writes, such as
by using read-modify-write operations on a larger location to write a smaller datum,
writing a value to memory that the program could not have written, or writing a
just-read value back to the location it was read from, if that location could have
been overwritten by another agent after the read.)
Possible read values must be non-empty: Program transformations cannot cause
the possible read values of a shared memory read to become empty.
(Counterintuitively, this rule in effect restricts transformations on writes, because
writes have force in memory model insofar as to be read
by read events. For example, writes may be moved and coalesced and sometimes
reordered between two seq-cst operations, but the
transformation may not remove every write that updates a location; some write must
be preserved.)
Examples of transformations that remain valid are: merging multiple non-atomic reads from the
same location, reordering non-atomic reads, introducing speculative non-atomic reads,
merging multiple non-atomic writes to the same location, reordering non-atomic writes to
different locations, and hoisting non-atomic reads out of loops even if that affects
termination. Note in general that aliased TypedArrays make it hard to prove that locations
are different.
Note 3
The following are guidelines for ECMAScript implementers generating machine code for shared
memory accesses.
For architectures with memory models no weaker than those of ARM or Power, non-atomic stores
and loads may be compiled to bare stores and loads on the target architecture. Atomic stores
and loads may be compiled down to instructions that guarantee sequential consistency. If no
such instructions exist, memory barriers are to be employed, such as placing barriers on
both sides of a bare store or load. Read-modify-write operations may be compiled to
read-modify-write instructions on the target architecture, such as
LOCK-prefixed instructions on x86, load-exclusive/store-exclusive instructions
on ARM, and load-link/store-conditional instructions on Power.
Specifically, the memory model is intended to allow code
generation as follows.
Every atomic operation in the program is assumed to be necessary.
Atomic operations are never rearranged with each other or with non-atomic operations.
Functions are always assumed to perform atomic operations.
Atomic operations are never implemented as read-modify-write operations on larger data,
but as non-lock-free atomics if the platform does not have atomic operations of the
appropriate size. (We already assume that every platform has normal memory access
operations of every interesting size.)
Naive code generation uses these patterns:
Regular loads and stores compile to single load and store instructions.
Lock-free atomic loads and stores compile to a full (sequentially consistent) fence, a
regular load or store, and a full fence.
Lock-free atomic read-modify-write accesses compile to a full fence, an atomic
read-modify-write instruction sequence, and a full fence.
Non-lock-free atomics compile to a spinlock acquire, a full fence, a series of
non-atomic load and store instructions, a full fence, and a spinlock release.
That mapping is correct so long as an atomic operation on an address range does not race with
a non-atomic write or with an atomic operation of different size. However, that is all we
need: the memory model effectively demotes the atomic
operations involved in a race to non-atomic status. On the other hand, the naive mapping is
quite strong: it allows atomic operations to be used as sequentially consistent fences,
which the memory model does not actually guarantee.
Local improvements to those basic patterns are also allowed, subject to the constraints of
the memory
model. For example:
There are obvious platform-dependent improvements that remove redundant fences. For
example, on x86 the fences around lock-free atomic loads and stores can always be
omitted except for the fence following a store, and no fence is needed for lock-free
read-modify-write instructions, as these all use LOCK-prefixed
instructions. On many platforms there are fences of several strengths, and weaker fences
can be used in certain contexts without destroying sequential consistency.
Most modern platforms support lock-free atomics for all the data sizes required by
ECMAScript atomics. Should non-lock-free atomics be needed, the fences surrounding the
body of the atomic operation can usually be folded into the lock and unlock steps. The
simplest solution for non-lock-free atomics is to have a single lock word per
SharedArrayBuffer.
There are also more complicated platform-dependent local improvements, requiring some
code analysis. For example, two back-to-back fences often have the same effect as a
single fence, so if code is generated for two atomic operations in sequence, only a
single fence need separate them. On x86, even a single fence separating atomic stores
can be omitted, as the fence following a store is only needed to separate the store from
a subsequent load.
Annex B (normative) Additional ECMAScript
Features for Web Browsers
The ECMAScript language syntax and semantics defined in this annex are required when the ECMAScript
host is a web browser. The content
of this annex is normative but optional if the ECMAScript host is not a web browser.
Note
This annex describes various legacy features and other characteristics of web browser ECMAScript
hosts. All of the language
features and behaviours specified in this annex have one or more undesirable characteristics and
in the absence of legacy usage would be removed from this specification. However, the usage of
these features by large numbers of existing web pages means that web browsers must continue to
support them. The specifications in this annex define the requirements for interoperable
implementations of these legacy features.
These features are not considered part of the core ECMAScript language. Programmers should not
use or assume the existence of these features and behaviours when writing new ECMAScript code.
ECMAScript implementations are discouraged from implementing these features unless the
implementation is part of a web browser or is required to run the same legacy ECMAScript code
that web browsers encounter.
B.1 Additional Syntax
B.1.1 HTML-like Comments
The syntax and semantics of 12.4 is extended as follows except that this
extension is not allowed when parsing source text using the goal
symbolModule:
The syntax of 22.2.1 is modified and extended as follows. These
changes introduce ambiguities that are broken by the ordering of grammar productions and by
contextual information. When parsing using the following grammar, each alternative is considered
only if previous production alternatives do not match.
This alternative pattern grammar and semantics only changes the syntax and semantics of BMP
patterns. The following grammar extensions include productions parameterized with the
[UnicodeMode] parameter. However, none of these extensions change the syntax of Unicode patterns
recognized when parsing with the [UnicodeMode] parameter present on the goal
symbol.
Return the CharSet containing the single
character \ U+005C (REVERSE SOLIDUS).
Note
This production can only be reached from the sequence
\c within a character class where it is not followed by an acceptable
control character.
B.1.2.8.1 CharacterRangeOrUnion ( rer,
A, B )
The abstract operation CharacterRangeOrUnion takes arguments rer (a RegExp
Record), A (a CharSet), and B
(a CharSet) and returns a CharSet. It performs the following steps
when called:
The abstract operation ParsePattern takes arguments
patternText (a sequence of Unicode code points), u (a Boolean), and
v (a Boolean). It performs the following steps when called:
If v is true and u is
true, then
Let parseResult be a List
containing one or more SyntaxError objects.
Else if v is true, then
Let parseResult be ParseText(patternText,
Pattern[+UnicodeMode,
+UnicodeSetsMode,
+NamedCaptureGroups]).
Else if u is true, then
Let parseResult be ParseText(patternText,
Pattern[+UnicodeMode,
~UnicodeSetsMode,
+NamedCaptureGroups]).
Else,
Let parseResult be ParseText(patternText,
Pattern[~UnicodeMode,
~UnicodeSetsMode,
~NamedCaptureGroups]).
This function is a property of the global object. It computes a new version of
a String value in which certain code units have been replaced by a hexadecimal escape
sequence.
When replacing a code unit of numeric value less than or equal to 0x00FF, a two-digit escape
sequence of the form %xx is used. When replacing a code unit of
numeric value strictly greater than 0x00FF, a four-digit escape sequence of the form
%uxxxx is used.
The encoding is partly based on the encoding described in RFC 1738, but the entire
encoding specified in this standard is described above without regard to the
contents of RFC 1738. This encoding does not reflect changes to RFC 1738 made by RFC
3986.
B.2.1.2 unescape ( string )
This function is a property of the global object. It computes a new version of
a String value in which each escape sequence of the sort that might be introduced by the
escape function is replaced with the code unit that it represents.
B.2.2 Additional Properties of the String.prototype Object
B.2.2.1 String.prototype.substr ( start,
length )
This method returns a substring of the result of converting the
this value to a String, starting from index start and running
for length code units (or through the end of the String if length is
undefined). If start is negative, it is treated as sourceLength + start where
sourceLength is the length of the String. The result is a String value,
not a String object.
Return the substring of S from
intStart to intEnd.
Note
This method is intentionally generic; it does not require that its
this value be a String object. Therefore it can be transferred to
other kinds of objects for use as a method.
B.2.2.2 String.prototype.anchor ( name )
This method performs the following steps when called:
Let escapedV be the String value that is the same as
V except that each occurrence of the code unit 0x0022
(QUOTATION MARK) in V has been replaced with the six code
unit sequence """.
The property "trimStart" is preferred. The
"trimLeft" property is provided principally for compatibility
with old code. It is recommended that the "trimStart" property be
used in new ECMAScript code.
The initial value of the "trimLeft" property is
%String.prototype.trimStart%, defined in 22.1.3.34.
B.2.2.16 String.prototype.trimRight ( )
Note
The property "trimEnd" is preferred. The
"trimRight" property is provided principally for compatibility
with old code. It is recommended that the "trimEnd" property be
used in new ECMAScript code.
The initial value of the "trimRight" property is
%String.prototype.trimEnd%, defined in 22.1.3.33.
B.2.3 Additional Properties of the Date.prototype Object
B.2.3.1 Date.prototype.getYear ( )
Note
The getFullYear method is preferred for nearly all purposes, because it
avoids the “year 2000 problem.”
This method performs the following steps when called:
This method completely reinitializes the this value RegExp with a
new pattern and flags. An implementation may interpret use of this method as an
assertion that the resulting RegExp object will be used multiple times and hence is
a candidate for extra optimization.
B.3 Other Additional Features
B.3.1 Labelled Function Declarations
Prior to ECMAScript 2015, the specification of LabelledStatement did not allow for the
association of a statement label with a FunctionDeclaration. However, a labelled
FunctionDeclaration was
an allowable extension for non-strict code and most browser-hosted
ECMAScript implementations supported that extension. In ECMAScript 2015 and later, the grammar
production for LabelledStatement permits use of FunctionDeclaration as a
LabelledItem but 14.13.1
includes an Early Error rule that produces a Syntax Error if that occurs. That rule is modified
with the addition of the highlighted text:
B.3.2 Block-Level Function Declarations Web Legacy Compatibility
Semantics
Prior to ECMAScript 2015, the ECMAScript specification did not define the occurrence of a FunctionDeclaration as an
element of a Block statement's
StatementList. However,
support for that form of FunctionDeclaration was an allowable
extension and most browser-hosted ECMAScript implementations permitted them. Unfortunately, the
semantics of such declarations differ among those implementations. Because of these semantic
differences, existing web ECMAScript source text that uses Block level function declarations is only
portable among browser implementations if the usage only depends upon the semantic intersection
of all of the browser implementations for such declarations. The following are the use cases
that fall within that intersection semantics:
A function is declared and only referenced within a single block.
One or more FunctionDeclarations whose
BindingIdentifier is the name
f occur within the function code of an enclosing function g
and that declaration is nested within a Block.
No other declaration of f that is not a var declaration
occurs within the function code of g.
A function is declared and possibly used within a single Block but also referenced by an inner function
definition that is not contained within that same Block.
One or more FunctionDeclarations whose
BindingIdentifier is the name
f occur within the function code of an enclosing function g
and that declaration is nested within a Block.
No other declaration of f that is not a var declaration
occurs within the function code of g.
There is at least one occurrence of f as an IdentifierReference within
another function h that is nested within g and no other
declaration of f shadows the references to f from within
h.
All invocations of h occur after the declaration of f has been
evaluated.
A function is declared and possibly used within a single block but also referenced within
subsequent blocks.
One or more FunctionDeclaration whose
BindingIdentifier is the name
f occur within the function code of an enclosing function g
and that declaration is nested within a Block.
No other declaration of f that is not a var declaration
occurs within the function code of g.
There is at least one occurrence of f as an IdentifierReference within the
function code of g that lexically follows the Block containing the declaration of
f.
The first use case is interoperable with the semantics of Block level function declarations provided by ECMAScript
2015. Any pre-existing ECMAScript source text that employs that use case
will operate using the Block level function declarations semantics defined by clauses 10, 14, and
15.
ECMAScript 2015 interoperability for the second and third use cases requires the following
extensions to the clause 10,
clause 15, clause
19.2.1 and
clause 16.1.7 semantics.
If an ECMAScript implementation has a mechanism for reporting diagnostic warning messages, a
warning should be produced when code contains a FunctionDeclaration for which these
compatibility semantics are applied and introduce observable differences from non-compatibility
semantics. For example, if a var binding is not introduced because its introduction would create
an early
error, a warning message should not be produced.
B.3.2.1 Changes to FunctionDeclarationInstantiation
NOTE: A var binding for F is only instantiated
here if it is neither a VarDeclaredName, the name of a
formal parameter, or another FunctionDeclaration.
If instantiatedVarNames does not contain
F and F is not
"arguments", then
It is a Syntax Error if the LexicallyDeclaredNames
of StatementList
contains any duplicate entries, unless IsStrict(this production) is
false and the duplicate entries are only bound by
FunctionDeclarations.
It is a Syntax Error if the LexicallyDeclaredNames
of CaseBlock contains any
duplicate entries, unless IsStrict(this production) is
false and the duplicate entries are only bound by
FunctionDeclarations.
The Block of a Catch clause may contain
var declarations that bind a name that is also bound by the CatchParameter. At
runtime, such bindings are instantiated in the VariableDeclarationEnvironment. They do
not shadow the same-named bindings introduced by the CatchParameter and hence the Initializer for such
var declarations will assign to the corresponding catch parameter rather
than the var binding.
This modified behaviour also applies to var and function declarations
introduced by direct
eval calls contained within the Block of a Catch clause. This change is accomplished by modifying
the algorithm of 19.2.1.3 as follows:
Objects with an [[IsHTMLDDA]] internal slot are never created by
this specification. However, the document.all
object in web browsers is a host-definedexotic object with this slot
that exists for web compatibility purposes. There are no other known examples of this
type of object and implementations should not create any with the exception of
document.all.
Assignment to an undeclared identifier or otherwise unresolvable reference does not create a
property in the global object. When a simple assignment occurs
within strict
mode code, its LeftHandSideExpression must not evaluate to
an unresolvable Reference. If it does a ReferenceError exception is thrown
(6.2.5.6). The
LeftHandSideExpression
also may not be a reference to a data property with the attribute value { [[Writable]]: false }, to an accessor
property with the attribute value { [[Set]]:
undefined }, nor to a non-existent property of an object whose [[Extensible]] internal slot is false. In these cases a
TypeError exception is thrown (13.15).
Arguments objects for strict functions do not dynamically share their
array-indexed
property values with the corresponding formal parameter bindings of their functions. (10.4.4).
For strict
functions, if an arguments object is created the binding of the local
identifier arguments to the arguments object is immutable and hence may not be the
target of an assignment expression. (10.2.11).
Strict mode eval code cannot instantiate variables or functions in the variable environment of the
caller to eval. Instead, a new variable environment is created and that environment is used for
declaration binding instantiation for the eval code (19.2.1).
If this is evaluated within strict mode code, then the
this value is not coerced to an object. A this value of either
undefined or null is not converted to the global
object and primitive values are not converted to wrapper objects. The
this value passed via a function call (including calls made using
Function.prototype.apply and Function.prototype.call) do not coerce the
passed this value to an object (10.2.1.2, 20.2.3.1, 20.2.3.3).
When a delete operator occurs within strict mode code, a
SyntaxError is thrown if its UnaryExpression is a direct reference to a
variable, function argument, or function name (13.5.1.1).
When a delete operator occurs within strict mode code, a
TypeError is thrown if the property to be deleted has the attribute { [[Configurable]]: false } or otherwise cannot be deleted
(13.5.1.2).
An implementation may not extend, beyond that defined in this specification, the meanings within
strict
functions of properties named "caller" or
"arguments" of function instances.
Preparation steps before, and cleanup steps after, invocation of JobAbstract Closures. See 9.5.
D.5 Internal Methods of Exotic Objects
Any of the essential internal methods in Table 4 for any
exotic
object not specified within this specification.
D.6 Built-in Objects and Methods
Any built-in objects and methods not defined within this specification, except as restricted in
17.1.
Annex E (informative) Corrections and
Clarifications in ECMAScript 2015 with Possible Compatibility Impact
9.1.1.4.14-9.1.1.4.17 Edition 5 and 5.1 used a
property existence test to determine whether a global object property corresponding to a new global
declaration already existed. ECMAScript 2015 uses an own property existence test. This corresponds to
what has been most commonly implemented by web browsers.
10.4.2.1: The
5th Edition moved the capture of the current array length prior to the integer conversion of the
array index or new
length value. However, the captured length value could become invalid if the conversion process has the
side-effect of changing the array length. ECMAScript 2015 specifies that the current array length must
be captured after the possible occurrence of such side-effects.
21.4.1.31: Previous
editions permitted the TimeClip abstract operation to return either
+0𝔽 or -0𝔽 as the representation of a 0
time value. ECMAScript 2015 specifies that
+0𝔽 always returned. This means that for ECMAScript 2015 the time value of a Date is never observably
-0𝔽 and methods that return time values never return
-0𝔽.
21.4.1.32: If a UTC offset representation is
not present, the local time zone is used. Edition 5.1 incorrectly stated that a missing time zone should
be interpreted as "z".
21.4.4.36: If the year cannot be
represented using the Date Time String Format specified in 21.4.1.32 a RangeError exception
is thrown. Previous editions did not specify the behaviour for that case.
21.4.4.41: Previous editions did not specify
the value returned by Date.prototype.toString when the time value is NaN.
ECMAScript 2015 specifies the result to be the String value "Invalid Date".
22.2.4.1, 22.2.6.13.1: Any LineTerminator code points in
the value of the "source" property of a RegExp instance must be expressed using an
escape sequence. Edition 5.1 only required the escaping of /.
22.2.6.8, 22.2.6.11: In previous editions,
the specifications for String.prototype.match and String.prototype.replace was
incorrect for cases where the pattern argument was a RegExp value whose global flag is set.
The previous specifications stated that for each attempt to match the pattern, if lastIndex
did not change, it should be incremented by 1. The correct behaviour is that lastIndex
should be incremented by 1 only if the pattern matched the empty String.
23.1.3.30: Previous editions did not specify how
a NaN value returned by a comparator was interpreted by
Array.prototype.sort. ECMAScript 2015 specifies that such as value is treated as if
+0𝔽 was returned from the comparator. ECMAScript 2015 also
specifies that ToNumber
is applied to the result returned by a comparator. In previous editions, the effect of a
comparator result that is not a Number
value was implementation-defined. In practice,
implementations call ToNumber.
Annex F (informative) Additions and Changes
That Introduce Incompatibilities with Prior Editions
6.2.5: In ECMAScript 2015,
Function calls are not allowed to return a Reference Record.
9.3: In
ECMAScript 2018, Template objects are canonicalized based on Parse Node (source location),
instead of across all occurrences of that template literal or tagged template in a Realm in previous editions.
12.2: In
ECMAScript 2016, Unicode 8.0.0 or higher is mandated, as opposed to ECMAScript 2015 which mandated
Unicode 5.1. In particular, this caused U+180E MONGOLIAN VOWEL SEPARATOR, which was in the
Space_Separator (Zs) category and thus treated as whitespace in ECMAScript
2015, to be moved to the Format (Cf) category (as of Unicode 6.3.0). This
causes whitespace-sensitive methods to behave differently. For example,
"\u180E".trim().length was 0 in previous editions, but 1 in
ECMAScript 2016 and later. Additionally, ECMAScript 2017 mandated always using the latest version of the
Unicode Standard.
12.7: In ECMAScript 2015, the valid code points
for an IdentifierName are specified
in terms of the Unicode properties “ID_Start” and “ID_Continue”. In previous editions, the valid IdentifierName or Identifier code points were specified by
enumerating various Unicode code point categories.
12.10.1: In ECMAScript 2015,
Automatic Semicolon Insertion adds a semicolon at the end of a do-while statement if the semicolon is
missing. This change aligns the specification with the actual behaviour of most existing
implementations.
13.2.5.1: In
ECMAScript 2015, it is no longer an early error to have duplicate property names in Object
Initializers.
13.15.1: In
ECMAScript 2015, strict mode code containing an assignment to an
immutable binding such as the function name of a FunctionExpression does not produce an early error. Instead it
produces a runtime error.
14.2: In ECMAScript 2015, a
StatementList beginning with the
token let followed by the input elements LineTerminator then Identifier is the start of a LexicalDeclaration. In previous editions, automatic
semicolon insertion would always insert a semicolon before the Identifier input element.
14.7: In ECMAScript 2015, if the (
token of a for statement is immediately followed by the token sequence let [ then the
let is treated as the start of a LexicalDeclaration. In previous editions such a
token sequence would be the start of an Expression.
14.7: In ECMAScript 2015, if the ( token of a
for-in statement is immediately followed by the token sequence let [ then the
let is treated as the start of a ForDeclaration. In previous editions such a token
sequence would be the start of an LeftHandSideExpression.
14.7: Prior to ECMAScript 2015, an
initialization expression could appear as part of the VariableDeclaration that precedes the
inkeyword. In ECMAScript 2015, the ForBinding in that same position does not
allow the occurrence of such an initializer. In ECMAScript 2017, such an initializer is permitted only
in non-strict
code.
14.15: In
ECMAScript 2015, it is an early
error for a Catch
clause to contain a var declaration for the same Identifier that appears as the Catch clause parameter. In previous editions, such a variable
declaration would be instantiated in the enclosing variable environment but the declaration's Initializer value would be assigned to the
Catch parameter.
14.15,
19.2.1.3: In ECMAScript 2015, a runtime
SyntaxError is thrown if a Catch clause evaluates a non-strict direct eval
whose eval code includes a var or FunctionDeclaration declaration that binds
the same Identifier that appears as the
Catch clause parameter.
15.4.5 In ECMAScript
2015, the function
objects that are created as the values of the [[Get]] or
[[Set]] attribute of accessor properties in an ObjectLiteral are not constructor functions and they do not have a
"prototype" own property. In the previous edition, they were constructors and had a
"prototype" property.
20.1.2.6:
In ECMAScript 2015, if the argument to Object.freeze is not an object it is treated as if
it was a non-extensible ordinary object with no own properties. In the previous
edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.8: In ECMAScript 2015, if the
argument to Object.getOwnPropertyDescriptor is not an object an attempt is made to coerce
the argument using ToObject. If the coercion is successful the result is used
in place of the original argument value. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.10: In ECMAScript 2015, if the
argument to Object.getOwnPropertyNames is not an object an attempt is made to coerce the
argument using ToObject. If the coercion is successful the result is used
in place of the original argument value. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.12: In ECMAScript 2015, if the argument
to Object.getPrototypeOf is not an object an attempt is made to coerce the argument using
ToObject. If the
coercion is successful the result is used in place of the original argument value. In the previous
edition, a non-object argument always causes a TypeError to be thrown.
20.1.2.16: In ECMAScript 2015, if the argument to
Object.isExtensible is not an object it is treated as if it was a non-extensible ordinary object
with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.17: In ECMAScript 2015, if the argument to
Object.isFrozen is not an object it is treated as if it was a non-extensible ordinary object
with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.18: In ECMAScript 2015, if the argument to
Object.isSealed is not an object it is treated as if it was a non-extensible ordinary object
with no own properties. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.19: In
ECMAScript 2015, if the argument to Object.keys is not an object an attempt is made to
coerce the argument using ToObject. If the coercion is successful the result is used
in place of the original argument value. In the previous edition, a non-object argument always causes a
TypeError to be thrown.
20.1.2.20: In ECMAScript 2015, if the
argument to Object.preventExtensions is not an object it is treated as if it was a
non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument
always causes a TypeError to be thrown.
20.1.2.22: In
ECMAScript 2015, if the argument to Object.seal is not an object it is treated as if it was
a non-extensible ordinary
object with no own properties. In the previous edition, a non-object argument
always causes a TypeError to be thrown.
20.2.3.2: In ECMAScript 2015, the [[Prototype]] internal slot of a bound function is set to the [[GetPrototypeOf]] value of its target function. In the previous edition, [[Prototype]] was always set to %Function.prototype%.
20.2.4.1: In ECMAScript 2015, the
"length" property of function instances is configurable. In previous editions it was
non-configurable.
21.4.4 In ECMAScript 2015,
the Date prototype object is not
a Date instance. In previous editions it was a Date instance whose TimeValue was NaN.
22.1.3.12 In ECMAScript 2015, the
String.prototype.localeCompare function must treat Strings that are canonically equivalent
according to the Unicode Standard as being identical. In previous editions implementations were
permitted to ignore canonical equivalence and could instead use a bit-wise comparison.
22.1.3.28 and 22.1.3.30 In ECMAScript 2015,
lowercase/upper conversion processing operates on code points. In previous editions such the conversion
processing was only applied to individual code units. The only affected code points are those in the
Deseret block of Unicode.
22.1.3.32 In ECMAScript 2015, the
String.prototype.trim method is defined to recognize white space code points that may exist
outside of the Unicode BMP. However, as of Unicode 7 no such code points are defined. In previous
editions such code points would not have been recognized as white space.
22.2.4.1 In ECMAScript 2015, If the
pattern argument is a RegExp instance and the flags argument is not
undefined, a new RegExp instance is created just like pattern except that
pattern's flags are replaced by the argument flags. In previous editions a
TypeError exception was thrown when pattern was a RegExp instance and
flags was not undefined.
22.2.6 In ECMAScript 2015,
the RegExp prototype object is
not a RegExp instance. In previous editions it was a RegExp instance whose pattern is the empty String.
25.4.15:
In ECMAScript 2019, Atomics.wake has been renamed to Atomics.notify to prevent
confusion with Atomics.wait.
27.1.6.4, 27.6.3.6: In ECMAScript 2019, the number of
Jobs enqueued by await
was reduced, which could create an observable difference in resolution order between a
then() call and an await expression.
Bibliography
IEEE 754-2019: IEEE Standard for Floating-Point Arithmetic.
Institute of Electrical and Electronic Engineers, New York (2019)
Note
There are no normative changes between IEEE 754-2008 and IEEE 754-2019 that affect the
ECMA-262 specification.
This specification is authored on GitHub in a plaintext
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Prior editions of this specification were authored using Word—the Ecmarkup source text that formed the
basis of this edition was produced by converting the ECMAScript 2015 Word document to Ecmarkup using an
automated conversion tool.
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